A UK Parliamentary by-election takes place when a seat in the House of Commons becomes vacant between general elections.
Why are by-elections held?
A by-election is held if a seat becomes vacant during the lifetime of a Parliament, either when an MP resigns from the House of Commons, 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.
What is '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 moved in the House of Commons Chamber "that the Speaker do issue his Warrant to the Clerk of the Crown to make out a new Writ for the electing of a Member to serve in this present Parliament for the constituency of .... in the room of ....".
The Speaker puts the question to the House for MPs to decide whether or not to agree to the motion.
If the motion is agreed it becomes an Order for the Speaker to issue his Warrant to the Clerk of the Crown who then sends the writ to the relevant Returning Officer.
What is the timetable for by-elections?
A new Writ is usually 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 a number of by-elections can take place on the same day.
The by-election timetable is between 21 and 27 working days from the issuing of the writ. The House of Commons Library have produced a briefing paper giving background information and a model by-election timetable.
Find out more about previous by-elections.