A UK parliamentary by-election happens when a seat in the House of Commons becomes vacant between general elections.
Why are by-elections held?
A by-election is held when a seat becomes vacant. This can happen when an MP:
- resigns or dies
- is declared bankrupt
- is unable to carry out their duties due to mental illness
- is convicted of a serious criminal offence.
A by-election does not have to take place if an MP changes political party.
Until an election, an MP of the same party in a neighbouring constituency manages constituency matters.
What is 'moving the writ'?
The Chief Whip of the political party whose MP held the vacant seat starts the process of a by-election.
"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 MPs to decide whether to agree to the motion.
If MPs agree it becomes an Order for the Speaker. The Speaker then issues a Warrant to the Clerk of the Crown who then sends the writ to the Returning Officer.
What is the timetable for by-elections?
A new Writ is usually issued within three months of the vacancy. There have been a few times when seats remained vacant longer than six months. Seats will be left vacant towards the end of a Parliament. They are then filled at the general election.
If there are many vacant seats 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.
Find out more about previous by-elections.
- Commons Library paper on by-elections 2019 -
- Commons Library paper on by-elections 2017-19
- Commons Library paper on by-elections 2015-17
- Commons Library paper on by-elections 2010-15
- Commons Library paper on by-elections 2005-10
- Commons Library paper on by-elections 2001-05
- UK by-elections since 1945: Commons Library paper
- UK by-elections held more than 3 months after vacancy: Commons Library paper