A by-election takes place when a seat in the House of Commons becomes vacant between general elections.

Reasons for by-elections

A seat becomes vacant during the lifetime of a Parliament either when an MP resigns from Parliament, for example to take up a job which by law cannot be done by an MP, or because an MP has died. The law also allows a seat to be declared vacant because of a Member's bankruptcy, mental illness or conviction for a serious criminal offence.

A by-election does not automatically take place if an MP changes political party.

Until a new MP is elected, constituency matters are handled by an MP of the same party in a neighbouring constituency.

'Moving the Writ' 

Traditionally the Chief Whip of the political party whose MP held the vacant seat will begin the procedure for a by-election. This is known as 'moving the Writ' and takes the form of a motion in the House of Commons.

By-election timetable 

A new Writ is moved within three months of the vacancy occurring. There have been a few instances of seats remaining vacant longer than six months before a by-election was called. Seats have also been left vacant towards the end of a Parliament to be filled at the general election.

If there are several vacant seats then a number of by-elections can take place on the same day.

Related information

Writs of Election: these are official notices, issued by the Clerk of the Crown, to Acting Returning Officers telling them that an election is to be held in their constituency.

Constituency: this is a geographical area of the UK which is represented by one MP in the House of Commons.

Recent by-election results