This is a list of parliamentary by-elections in the United Kingdom held between 1900 and 1918, with the names of the incumbent and victor and their respective parties. Where a political party gained a seat at the election, the result is highlighted: pink for Labour (including Labour Representation Committee), light blue for Conservative (including Liberal Unionist, Irish Unionist and Scottish Unionist), orange for Liberal (including Liberal-Labour), green for Sinn Féin, light green for the Irish Parliamentary Party and grey for any other gain.
Where the cause of by-election is given as "resignation" or "seeks re-election", this indicates that the incumbent was appointed on his 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.
The Succession to the Crown Act 1707 required ministers to seek re-election to the House of Commons on their appointment to office. The Re-Election of Ministers Act 1919 ended the necessity to seek re-election within nine months of a general election, and the Re-Election of Ministers Act (1919) Amendment Act 1926 ended the practice in all other cases.