This is a list of parliamentary by-elections in the United Kingdom held between 1931 and 1950, with the names of the incumbent and victor and their respective parties. Where seats changed political party at the election, the result is highlighted: red for a Labour gain, blue for a Conservative gain, orange for a Liberal gain, yellow for an SNP gain and grey for any other gain. A grand total of 333 by-elections were held during this period.

Resignations

Where the cause of by-election is given as "resignation" or "seeks re-election", this indicates that the incumbent was appointed on his or her own request to an "office of profit under the Crown", either the Steward of the Chiltern Hundreds or the Steward of the Manor of Northstead. These appointments are made as a constitutional device for leaving the House of Commons, whose Members are not permitted to resign.

By-elections

{| class="wikitable"

1945–1950 Parliament

!By-election !! Date !! Incumbent !! colspan=2 | Party !! Winner !! colspan=2 | Party !! Cause

1935–1945 Parliament

This Parliament's life was extended by annual Prolongation of Parliament Acts for the duration of the Second World War. By-elections continued to fill vacancies. An electoral truce was negotiated between the Conservative, Labour, Liberal, National Liberal and National Labour parties, and National independent MPs that they would not contest by-elections which another party held (although there were a few occasions when a National party would step aside from a vacancy in favour of a National independent, usually a government minister). However many independents stood, including some party members who disagreed with the truce. The Common Wealth Party was formed in part with a view to contesting wartime by-elections. A total of 219 by-elections were held during this period. !By-election !! Date !! Incumbent !! colspan=2 | Party !! Winner !! colspan=2 | Party !! Cause

1931–1935 Parliament

!By-election !! Date !! Incumbent !! colspan=2 | Party !! Winner !! colspan=2 | Party !! Cause