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 seats changed political party at the election, the result is highlighted: pink for a Labour gain, light blue for a Conservative (including Liberal Unionist and Irish Unionist) gain, orange for a Liberal (including Liberal-Labour) gain, green for a Sinn Féin gain, light green for an Irish Parliamentary Party gain and grey for any other gain.