Parliamentary by-elections in the United Kingdom occur when a Member of Parliament (MP) vacates a House of Commons seat (due to resignation, death, disqualification or expulsion) during the course of a parliament.

Scope of these records

Although the history of Parliament is much older, most of these records concern only the period since 1945. Earlier exceptional results are listed separately. Parliaments of England, Scotland, Ireland and the various unions of these Kingdoms had been assembled since the medieval period, though these bodies only gradually evolved to be democratically elected by the populace and records are incomplete. England and Wales had numerous "rotten boroughs" with tiny and tightly controlled electorates until the Reform Act of 1832. The most recent significant expansions of the electoral franchise were the Representation of the People Act 1918 which allowed some women to vote for the first time and greatly expanded the franchise of men, overall more than doubling the size of the electorate, and the Representation of the People (Equal Franchise) Act 1928 which expanded the franchise of women to be equal to that of men. Furthermore, there are various additional factors complicating comparisons between earlier results and modern cases. Among the most significant aspects of historical elections which are no longer present are: Since 1945, the legal and general political situation regarding by-elections has been broadly stable, allowing for meaningful comparison of records. These records include those from Northern Ireland. However, the politics of Northern Ireland is mostly separate from that of Great Britain so comparisons can be problematic.

Glossary

For comparison purposes the following definitions have been adopted.

Numerical records

For more information about what is meant by the term "swing", see Swing (United Kingdom)

Largest swings in percentage share of votes

Largest swings to an incumbent governing party

It is rare to see any swing towards the governing party in by-elections. However, there are some examples of it happening.

• BOLD indicates winning party

Largest fall in percentage share of vote

A party's share of the vote at a general election is not always matched at subsequent by-elections, but given the five-year maximum term of a Parliament, reductions of 20% or more are unusual. Those of 25% or more are listed below: In the 1934 Merthyr by-election the Independent Labour Party share dropped from 69.4% in the 1931 general election to 9.8% (a record 59.6% loss) losing the seat to the Labour Party. However, the 1931 election had no Labour Party candidate, and the MP, R. C. Wallhead, had previously been elected as a Labour candidate in prior elections, when the ILP was affiliated to Labour. Prior to his death, Wallhead joined the Labour Party, so this result could be classed as a Labour hold. The 1919 East Antrim by-election saw the Irish Unionist party face its first Unionist opposition in the seat since 1906 (in the 1918 general election the heavily unionist area gave the Irish Unionist 94.6% of the vote in a contest with a Sinn Féin candidate). An Independent Unionist candidate won the seat, with the Irish Unionist share dropping by 52.8% Worst results for other parties:

Largest increase in percentage share of vote

Largest winning share of the vote

Winning shares of the vote above 90%, since 1918:

Largest numerical majority overturned

Majorities over 9,000 votes overturned:

Lowest winning share of the vote

Winning shares of the vote below 35%, since 1918: The 1920 Stockport by-election, was held to elect two MPs. The winners' shares of the total vote were 25.6% and 25.1%. However, as each voter could cast two votes, the situation is not readily comparable to other by-elections in this period. At the 1909 Sheffield Attercliffe by-election, the winning candidate took only 27.5% of the vote.

Lowest share of the vote

Major parties

Major parties winning 2% or less share of votes cast in a by-election, since 1918: The worst Conservative performance was in the 1995 North Down by-election, where they took 2.1% of the votes cast. The 'continuing' Social Democratic Party (SDP) took 0.4% of the vote at both the 1990 Upper Bann by-election and the Bootle by-election the following week.

Candidates winning fewer than ten votes

Since 1918:

Smallest majorities

All majorities of less than 1,000 since the Second World War. Bold entries indicate a new record. Still smaller majorities have been recorded since 1918. The majority in the 1921 Penrith and Cockermouth by-election, was only 31 votes, and in the 1924 Westminster Abbey by-election it was 43 votes, while at the 1928 Carmarthen by-election it was 47 votes. At the 1892 Cirencester by-election a majority of 3 for the Unionists was overturned on petition, where it was found that both candidates had an equal number of votes. A fresh by-election was called, which was won by the Liberals. The 1830 Liverpool by-election saw a majority of 29 votes.

Turnout

Turnout is the percentage of registered electors who voted.

Highest turnout

The highest turnouts since 1918.

Turnout increased from general election

It is highly unusual for a by-election to attract a higher turnout in a seat than the previous general election.

Lowest turnout

During the Second World War the electoral register was not kept up to date despite significant population movements, especially in the London area (which contains all three constituencies in the first list below). Consequently, only those eligible to vote in the constituency at the outbreak of war were eligible to vote in the by-elections, and many of those were physically unable to vote, as they were located elsewhere; in addition the major parties did not compete against each other. The lowest turnout in peacetime since 1918 was 18.2% at the 2012 Manchester Central by-election. The lowest turnouts since 1918 have been: Turnouts of less than 30% since 1945 (bold indicates a new post-war record)

Most candidates

Under current UK electoral law there is no upper or lower limit for candidature numbers, with the only required stipulation being the valid nomination of ten electors from the constituency. By-elections often attract "fringe" or novelty candidates, single-issue candidates, or independents. As with nominations in a general election, candidates must pay a £500 deposit, which is only refunded if the candidate wins 5% of the votes cast. All by-elections with more than ten candidates are listed. Elections are listed in alphabetical order. Those that created a new record number appear in bold. In 2017, the countermanded poll in Manchester Gorton had 11 candidates.

Fewest candidates

Candidate records

Durable by-election candidates

Major parties

Former Labour cabinet minister Tony Benn contested no fewer than four by-elections during his career, topping the poll on each occasion: Bristol South East in 1950, 1961 and 1963, and Chesterfield in 1984. His first and last by-election victories were 33 years and 3 months apart. Former cabinet minister and European Commissioner Roy Jenkins fought two different by-elections for the Social Democratic Party only eight months apart. He narrowly failed in the 1981 Warrington by-election before winning the 1982 Glasgow Hillhead by-election. He had been first elected as a Labour MP almost 34 years previously in the 1948 Southwark Central by-election. Former Speaker of the House of Commons, Betty Boothroyd finally secured election at her third by-election attempt at the 1973 West Bromwich by-election. She had previously failed in the 1957 Leicester South East by-election and the 1968 Nelson and Colne by-election as well as the general elections of 1959 and 1970. John Bickley of UKIP contested three by-elections (all in Greater Manchester) within two years - Wythenshawe and Sale East in February 2014, Heywood and Middleton in October 2014 and Oldham West and Royton in December 2015. He was defeated on each occasion, coming closest in Heywood and Middleton where he lost by less than 700 votes. Bickley also contested Heywood and Middleton at the 2015 general election, making a total of four parliamentary elections contested in fewer than 24 months.

Minor parties and independents

Perennial fringe candidates include such personalities as Bill Boaks, who ran in 19 by-elections. His highest vote was at the 1982 Beaconsfield by-election with 99 votes. Screaming Lord Sutch was for most of his career the leader of the Official Monster Raving Loony Party, and competed in 34 by-elections (1 for the National Teenage Party) between 1963 and 1997. His highest vote total was 1,114 at the 1994 Rotherham by-election. Lindi St Clair of the Corrective Party contested eleven by-elections without success, her highest total being 216 votes as 'Lady Whiplash' at the 1990 Eastbourne by-election. Sutch's successor as leader of the Official Monster Raving Loony Party, Alan "Howling Laud" Hope, has, as of the 2023 Uxbridge and South Ruislip by-election, contested twenty-one by-elections. Under various ballot paper descriptions, David Bishop of the Church of the Militant Elvis label stood at seven by-elections, receiving 99 votes at the 2012 Corby by-election, an increase over his previous high of 93 at the 2011 Feltham and Heston by-election. On 23 January 1986, Wesley Robert Williamson (who changed his name to Peter Barry) stood in four simultaneous by-elections in Northern Ireland.

Pre-1945

Arthur Henderson was distinguished in being successful in no fewer than five by-elections in different seats, in Barnard Castle, Widnes, Newcastle upon Tyne East, Burnley, and Clay Cross. Joseph Gibbins is the only person in modern times to gain the same seat twice in two different by-elections. He triumphed for Labour in the 1924 and 1935 Liverpool West Toxteth by-elections. William O'Brien won four by-elections, in Mallow in 1883, North East Cork in 1887 and then Cork City in 1904 and 1914. On these last two occasions, he was re-elected having resigned the seat. Prime Minister Winston Churchill contested five by-elections in his long career: John Wilkes won the 1757 Aylesbury by-election, and was then elected in the Middlesex by-elections of February, March and April 1769, on each occasion being subsequently expelled from the House of Commons.

Former MPs making a comeback at a by-election

Notes: 1 by-election gain lost at the subsequent general election 2 by-election gain held at the subsequent general election

Former MPs failing in a by-election

Re-election of ministers

Until the Re-election of Ministers Acts 1919 and 1926 there were many cases of members having to seek re-election on appointment to ministerial office. In eight instances since 1900 they were unsuccessful:

Shortest-serving by-election victors

Note this list covers completed service only; it excludes any current MPs.

Since 1945

Notes

Pre-1945

Youngest by-election victors

Babies of the House elected at by-elections

See Baby of the House of Commons

Oldest by-election victors

Debuts in Parliament: Comebacks to Parliament: In defence of a previously held seat:

First women by-election victors

The first woman to be elected in a by-election was Nancy Astor, who succeeded her husband at the 1919 Plymouth Sutton by-election, becoming the first woman to take her seat in the House of Commons. The first woman to gain a seat in a by-election was Susan Lawrence who won the 1926 East Ham North by-election, although she had previously sat for the same seat between 1923 and 1924.1 The first woman to gain a seat ab initio in a by-election was Jennie Lee who won the 1929 North Lanarkshire by-election, at the same time becoming the first woman Baby of the House of Commons. Note 1 Mabel Philipson succeeded her husband at the 1923 Berwick-upon-Tweed by-election. He had been elected as a National Liberal Party candidate. She won as a Conservative so this could arguably be classed as the first gain by a woman.

First ethnic minority by-election victors

Whilst the first ethnic minority Members of Parliament were elected at general elections as early as the 1890s, it would be almost 100 years before one was returned at a by-election. The first ethnic minority candidate to be elected in a by-election was Ashok Kumar who gained the 1991 Langbaurgh by-election for Labour. The first by-election in which all three major-party candidates were from the ethnic minorities was the 2007 Ealing Southall by-election, held by Labour.

First by-election victors from specific religions

When the UK Parliament was established in 1801, non-Anglicans were prevented from taking their seats as MPs under the Test Act 1672. However, Methodists took communion at Anglican churches until 1795, and some continued to do so, and many Presbyterians were prepared to accept Anglican communion, thus ensuring that members of these creeds were represented in the Parliament. Some Unitarians were also elected. The first by-election victor (and first ever MP) to be an adherent of the Eastern Orthodox Church was The Honourable Frederick North who was elected in 1792 for Banbury (to succeed his brother who had entered the House of Lords), having converted to the faith the previous year. The first Roman Catholic by-election victor in the UK Parliament was Daniel O'Connell in the 1828 Clare by-election. He was not permitted to take his seat until the following year. The first atheist by-election victor was Charles Bradlaugh, at the 1881 Northampton by-election. As an atheist, Bradlaugh was not allowed to swear the Oath of Allegiance, and the by-election was re-run in 1882 and 1884. Both were also won by Bradlaugh, who eventually was able to take his seat after the 1885 general election.

Physically disabled by-election victors

Most physically disabled MPs in the history of the parliament entered in the intakes of general elections. Those known to have been disabled when entering parliament at by-elections are rarer and include:

By-elections losers awarded seats on disqualification of winner

Two or more former MPs contest by-election

1 Hilton Dawson, Thelma Walker, and Paul Williams 2 Conservative MPs David Davis and Walter Sweeney

Winner of previous election runs again

The usual causes of by-elections – death or resignation – mean that incumbent MPs are rarely candidates. Typically, this only occurs if an MP deliberately triggers a by-election to get a mandate for a party change, or to bring a particular issue to public attention. More rarely, it can occur if an election result is challenged in court or, since the Recall of MPs Act 2015, an MP chooses to run after a recall petition against them succeeds. Historically, such cases were more common due to ministerial by-elections (see ). Examples since 1945 include:

Frequency and duration records

Longest period without a by-election

The years 1998 and 2020 stand as the two in modern British history without any Westminster election. 1992, 1998, 2010 and 2020 are the four full calendar years in history without a single by-election.

Longest period between a vacancy arising and a by-election writ being moved

Longest period without a seat changing hands

The longest period without a seat changing hands in a by-election was the five years between the Conservative victories in the 1948 Glasgow Camlachie by-election and the 1953 Sunderland South by-election. During the short Parliaments of 1910, 1950-1 and 1974 no seats changed hands in a by-election.

Longest period between by-election gains for a party

The Liberal Party endured 29 years without a single by-election gain between the 1929 Holland with Boston by-election and the 1958 Torrington by-election. It did not win a single by-election in the thirteen years between holding the 1945 Middlesbrough West by-election and gaining Torrington. Until the 2008 Crewe and Nantwich by-election, the opposition Conservative Party had not gained a seat in almost 26 years, the last being the 1982 Mitcham and Morden by-election, which occurred during the unique circumstances of the Falklands War and the sitting Labour MP defecting to the Social Democratic Party and seeking re-election under his new party label. The Conservatives' last gain while in Opposition was 30 years previously at the 1978 Ilford North by-election. Labour's longest lean stretch was almost 18 years, between gaining the 1939 Brecon and Radnorshire by-election and the 1957 Lewisham North by-election.1 As of, the most recent gains for each currently active party were: Notes 1 The Labour Party were the official opposition in the Parliament elected in 1935, but after the major parties agreed an electoral truce on the outbreak of war in 1939, they did not contest any Conservative or Liberal seats for the remainder of the Parliament, a period of six years, and were members of the wartime coalition government between May 1940 and May 1945. 2 Notional gain: incumbent Conservative stood as UKIP. No UKIP candidate has ever defeated an incumbent of a different party 3 The UUP were also declared winners of the 1955 Mid Ulster by-election after the Sinn Féin candidate was disqualified, but the UUP candidate was also disqualified shortly after. 4 Sinn Féin have not gained a seat at a by-election since 1918. However, the Anti H-Block party, an Irish Republican group that merged into Sinn Féin, gained Fermanagh and South Tyrone in the April 1981 by-election ( ago).

Longest period between by-election holds for a party

The Conservatives did not successfully defend a single by-election in the eight years between their holds of the 1989 Richmond (Yorks) by-election and the 1997 Uxbridge by-election, losing a record 15 consecutive seats where they were the incumbents. By the time of the by-election in Uxbridge, the victor in Richmond, William Hague, had become leader of the Conservative Party. Labour's worst run was 4 consecutive by-election losses, which has occurred three times since 1945:

Longest period between by-election losses for a party

Between the 1988 Glasgow Govan by-election and the 2003 Brent East by-election, Labour successfully defended every seat it held at by-elections, for a total of 30 holds (not counting Falkirk West and West Bromwich West, represented by a Labour MP turned independent and a Labour speaker respectively and both won by Labour). The span of 14 years, 10 months, 14 days is the longest period without a by-election defeat for either of the two main parties. The Conservatives did not lose a seat between the 2000 Romsey by-election and the 2012 Corby by-election, a span of 12 years, 6 months, 19 days. However, they only defended 3 seats in that time. In terms of total number, their longest run of by-election holds was 51, between the 1945 Chelmsford by-election and the 1957 Lewisham North by-election, a span of 11 years, 9 months, 29 days. Since their formation, the Liberal Democrats have held every Lib Dem seat contested at a by-election, of which there have been 3. Including their predecessor parties, their most recent by-election loss was the 1982 Mitcham and Morden by-election, lost by the SDP ago. The SDP candidate had however defected from Labour – the last seat lost by either party that had been won at a previous election was the 1957 Carmarthen by-election, lost by the Liberals ago. Since 1982, the Liberal Democrats and predecessors together have defended 4; since 1957 they have defended 5 seats. By-elections in seats held by minor and nationalist parties are rare, and so most have never lost a seat. The SNP lost their first seat at the 2023 Rutherglen and Hamilton West by-election. The DUP and Plaid Cymru have defended but never lost a seat at a by-election, Sinn Féin have only lost seats by disqualification, and the UUP have never lost more than one seat in a row. No by-election has ever been called in an SDLP held seat.

Longest period without an opposition gain

For a period of 11 years, from the 1997 general election until the 2008 Crewe and Nantwich by-election, the principal opposition Conservative Party failed to register a by-election gain against the incumbent Labour Government. This is the longest period of such failure since records began, and more than twice the previous record of the five years it took the then Labour opposition to gain the 1957 Lewisham North by-election. Labour did however lose 3 seats to the Liberal Democrats in that period, their first being the 2003 Brent East by-election. The Labour Party, in its period in opposition starting in 2010, did not register a gain for over nine years, between the 2012 Corby by-election and the 2022 Wakefield by-election. In this period, the Conservative government lost three seats in by-elections where the former Conservative incumbent stood for a different party (2014 Clacton by-election and 2014 Rochester and Strood by-election, both lost to UKIP) or as an independent (2016 Richmond Park by-election, lost to the Liberal Democrats). Their first loss in a by-election without a sitting incumbent was to the Liberal Democrats at the 2019 Brecon and Radnorshire by-election. Apart from the brief parliaments of 1910, 1950-1 and 1974, the parliaments of 1951-5 and 1997-2001 are the only occasions when the Government did not lose a by-election.

Most by-elections in one day

The largest number of by-elections held on a single day occurred on 23 January 1986 when 15 simultaneous contests were held in Northern Ireland. The elections had been engineered by the incumbent Unionist parties as a protest against the Anglo-Irish Agreement of 1985. They intended the results to be interpreted as a referendum on the treaty. The elections were boycotted by the main Nationalist parties except in four seats where they had a reasonable prospect of victory. In the event, the Social Democratic and Labour Party gained one seat, Newry and Armagh, from the Ulster Unionist Party. Apart from the above example, it is common for UK mainland parties to schedule several by-elections on the same day. Motivations include attempting to divide opponents' resources and getting bad news (expected losses) out of the way. Since 1945, the largest number of simultaneous mainland by-elections has been 6, held on 16 November 1960. On four occasions, 5 by-elections have been held on the same day, most recently on 9 June 1994. Groupings of two or three are very common. Before November 2012, the last day on which three by-elections had been held was 23 November 2000. In November 2012 there were two such groupings of three (15 November and 29 November). The last time there were six by-elections in one calendar month was in June 1994.

Most by-election losses in one day

The largest number of by-elections lost on a single day is three, when the Labour party lost Acton, Dudley and Meriden on 28 March 1968, all to the Conservatives. Occasions since 1945 when two seats have fallen are:

Seats with more than one by-election in a single Parliament

Other seats with by-elections less than five years apart

By-election days

British Parliamentary elections are invariably held on a Thursday. The last by-election not held on a Thursday was the 1978 Hamilton by-election, held on Wednesday 31 May due to a World Cup opening match on the Thursday evening. Due to an administrative oversight, the 1973 Manchester Exchange by-election was held on Wednesday 27 June 1973. Prior to that, the last by-elections not held on a Thursday were the 1965 Saffron Walden by-election held on Tuesday 23 March, and the 1965 Roxburgh, Selkirk and Peebles by-election held the following day. Until the mid-1960s, it was common to hold by-elections on any day of the week (other than Sunday).

Countermanded poll

Very occasionally, a scheduled by-election may be overtaken by the calling of a general election and the dissolution of Parliament, in which case the poll is countermanded by the Returning Officer. There have been only three occasions since 1918: a by-election was scheduled to take place in Warwick and Leamington on 21 November 1923, but was cancelled by a dissolution of Parliament on 16 November. A by-election was scheduled to poll between 13 and 17 October 1924 in London University but was cancelled by a dissolution of Parliament on 9 October. In 2017 the Manchester Gorton by-election was cancelled by a Motion in the House of Commons following the calling of the 2017 United Kingdom general election.

Seats left vacant

Occasionally seats are left vacant for a substantial period. No by-election writ was moved for any seat held by Sinn Féin after the 1918 general election. Four Sinn Féin candidates were elected in two different seats and would have had to decline one of them if they had wanted to take their seats. They were Éamon de Valera (East Clare and East Mayo), Arthur Griffith (East Cavan and North West Tyrone), Eoin MacNeill (Londonderry City and National University of Ireland) and Liam Mellowes (East Galway and North Meath). By the end of the Parliament, the following Sinn Féin MPs had died without being replaced: Pierce McCan (East Tipperary) of influenza on 6 March 1919, Terence MacSwiney (Mid Cork) following a hunger strike in Brixton prison on 25 October 1920, Frank Lawless (North Dublin) as a result of a riding injury on 16 April 1922, Joseph McGuinness (Longford) on 31 May 1922, Cathal Brugha (Waterford) in action during the Irish Civil War on 7 July 1922, Harry Boland (South Roscommon) shot while being arrested on 2 August 1922, Arthur Griffith (East Cavan and North West Tyrone) on 12 August 1922, and Michael Collins (South Cork assassinated on 22 August 1922). In each case their seats were abolished in 1922 as a result of the establishment of the Irish Free State. Other than these cases the longest time a seat has been left vacant with no by-election held is when Dennis Vosper was elevated to the Peerage on 20 April 1964, and no writ was moved by the time Parliament was dissolved on 25 September 1964.

Causes of by-elections

By-elections prompted by assassination

By-elections prompted by suicide

Note

By-elections prompted by accidental death

By-elections prompted by posthumous election of MP

By-elections prompted by a successful recall petition

By-elections prompted by scandal

By-elections prompted to provide seat for seat-less personality

By-elections prompted by party disputes and defections

By-elections are ostensibly to vote for a 'person', not a 'party', meaning that a member switching parties mid-term is not cause for a by-election. However, some members do seek re-election under their new party as a point of principle.

By-elections resulting from Members resigning on principle

By-elections prompted by member's desire to contest another seat

By-elections caused by the previous result being declared void

By-elections prompted by disqualification of the sitting Member

By-elections prompted by expulsion from the House

By-elections prompted by lunacy

By-elections prompted by bankruptcy

By-elections prompted for miscellaneous reasons

By-elections prompted by death of member on wartime active service

Second World War

First World War

Miscellaneous records

Incumbents fall directly from first place to fourth place

Incumbents fall directly from first place to third place

Incumbent government gains seats

These records show the rare occasions when the government won a seat they had not won at the previous general election.

Principal Opposition loses seats

These records show the rare occasions when the official Opposition failed to hold on to a seat they had won at the previous General election.

By-election holds overturned at next general election

On rare occasions a party has failed to overturn an incumbent in the by-election yet has gone on to gain the seat at the subsequent general election.

By-election victors had not contested previous general election

It is unusual for a political party which has not contested a seat at a general election to take it at a subsequent by-election. Independent candidates are not included. Notes:

Additional victories by minor parties

In addition to the above section, other minor party successes include the following. For a complete list, see the list of minor party and independent MPs elected in the United Kingdom.

Minor parties' other strong performance

Minor parties without representation in the House of Commons which saved their deposit:

Victory from third or lower place

Incumbent party did not contest

Losers had been unopposed at previous election

Notes:

Major party did not run

Great Britain

Labour joined the Liberal Democrats and the Greens in not contesting the 2022 Southend West by-election, out of respect following the murder of the previous MP, Sir David Amess. The Conservatives declined to run a candidate in the 2016 Richmond Park by-election, instead backing Conservative incumbent Zac Goldsmith, who was designated as an Independent. The Conservatives, Liberal Democrats, UKIP, and Green Party declined to run candidates in the 2016 Batley and Spen by-election, due to the circumstances regarding the murder of the previous MP, Jo Cox. Neither the Liberal Democrat nor the Labour Party stood candidates in the 2008 Haltemprice and Howden by-election. The by-election was a single-issue election in regards to government security policy, in which the Liberal Democrats supported the Conservative candidate. The Conservative Party did not run a candidate in the 1963 Bristol South East by-election, the 1957 Carmarthen by-election, the 1948 Paisley by-election or the 1946 Ogmore by-election. The Labour Party did not run in the 1945 City of London by-election, the 1945 Kensington South by-election or the 1946 Combined English Universities by-election. Prior to 2008, the last by-election without an official Liberal Democrat, Liberal or SDP candidate had been the 1994 Newham North East by-election; the Lib Dems nominated a candidate, but he joined the Labour Party before the election. No official Liberal candidate was nominated for the 1980 Glasgow Central by-election, whilst no Liberal stood in either the 1973 Westhoughton by-election or the 1973 West Bromwich by-election, both held on 24 May 1973. The last Scottish by-elections without official Scottish National Party candidates were the 1965 Roxburgh, Selkirk and Peebles by-election and the 1964 Rutherglen by-election. Plaid Cymru did not stand a candidate for the 2019 Brecon and Radnorshire by-election, choosing instead to endorse the Liberal Democrat candidate in a "Stop Brexit" alliance. Prior to that, the last Welsh by-elections without official Welsh Nationalist candidates were the 1950 Abertillery by-election, the 1946 Pontypool by-election and the 1945 Monmouth by-election.

Northern Ireland

The more fluid nature of politics in Northern Ireland makes it harder to define all major parties. In addition many by-elections have not been contested by parties holding other seats in the House of Commons, whether due to agreements with other parties, poor organisation in the constituency or the particular circumstances on the by-election. However, for the period since 1981 (which saw the first by-elections in twelve years, during which time several major political realignments had occurred) the main parties are usually considered to be the Democratic Unionist Party, Sinn Féin, the Social Democratic and Labour Party (SDLP) and the Ulster Unionist Party. At the 2013 Mid Ulster by-election. a single "unity" candidate was backed by the withdrawal of the Democratic Unionist Party, Ulster Conservatives and Unionists and Traditional Unionist Voice. Prior to Mid Ulster in 2013, the most recent examples of by-elections without official Democratic Unionist candidates were the 1995 North Down by-election and the 1990 Upper Bann by-election. They also did not stand in the twelve seats held by other Unionist parties in the 15 by-elections in 1986. The last by-election without official candidates from either Sinn Féin or the SDLP was the 1995 North Down by-election. Both parties also declined to stand in the eleven Unionist majority seats in the 15 by-elections in 1986. The SDLP also did not contest either the April or August 1981 by-elections in Fermanagh and South Tyrone. The last by-elections without official Ulster Unionist candidates prior to Mid Ulster in 2013 were North Antrim, East Belfast, Mid Ulster and North Down in the 15 by-elections in 1986. The main British parties have generally not stood in seats in Northern Ireland. The by-election exceptions are the 1990 Upper Bann by-election (NI Conservatives and continuing SDP) and the 1995 North Down by-election (NI Conservatives). Prior to the 1970s the Ulster Unionists were effectively the local Conservatives, whilst the Liberals contested some but not all seats. The SDLP has traditionally seen itself as a "sister party" to the British Labour party, and its MPs usually accept the Labour whip in Parliament.

Miscellaneous notable results

It is unusual for one of the major parties to finish outside of the top three in England and Wales (or outside of the top four in Scotland). It is also unusual for the principal opposition party to suffer a significant reverse in its share of the vote or ranking. Notes 1Excluding the 1931 Westminster St George's by-election and the 1930 Paddington South by-election, which were essentially intra-Conservative contests, the previous worst result was, ironically, the 1930 Bromley by-election

By-elections having national significance

Firsts and lasts