A UK parliamentary by-election is held when a House of Commons seat becomes vacant between general elections.
Why does a seat become vacant?
Common reasons include the sitting MP dying or resigning. Vacancies can also arise following a successful recall petition or where an MP becomes ineligible to remain a Member.
An MP changing political party does not by itself cause a by-election.
What does “moving the writ” mean?
The formal process begins when the House of Commons agrees to issue a writ for a new election. By convention, a whip from the party that previously held the seat will usually initiate the motion.
How quickly is the election held?
There is no single fixed deadline from the moment a seat becomes vacant. Once the writ triggers the statutory election process, the parliamentary by-election timetable runs for 21 to 27 working days.
Is a by-election a national vote?
No. Only electors in the vacant constituency vote. The result changes the representation of that one constituency and the Commons seat totals accordingly.
Can by-elections tell us what will happen nationally?
They provide a real electoral result at a particular time and place, but the electorate, candidates, turnout and campaign context are local to that contest. A by-election result should therefore not automatically be treated as a national general election result.
Sources
UK Parliament: by-elections ↗ · Commons Library: by-election timetables ↗