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.
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.