Parliamentary by-elections in the United Kingdom occur when a Member of Parliament (MP) vacates a House of Commons seat (due to resignation, death, disqualification or expulsion) during the course of a parliament.
Although the history of Parliament is much older, most of these records concern only the period since 1945. Earlier exceptional results are listed separately.
Parliaments of England, Scotland, Ireland and the various unions of these Kingdoms had been assembled since the medieval period, though these bodies only gradually evolved to be democratically elected by the populace and records are incomplete. England and Wales had numerous "rotten boroughs" with tiny and tightly controlled electorates until the Reform Act of 1832. The most recent significant expansions of the electoral franchise were the Representation of the People Act 1918 which allowed some women to vote for the first time and greatly expanded the franchise of men, overall more than doubling the size of the electorate, and the Representation of the People (Equal Franchise) Act 1928 which expanded the franchise of women to be equal to that of men.
Furthermore, there are various additional factors complicating comparisons between earlier results and modern cases. Among the most significant aspects of historical elections which are no longer present are:
Since 1945, the legal and general political situation regarding by-elections has been broadly stable, allowing for meaningful comparison of records.
These records include those from Northern Ireland. However, the politics of Northern Ireland is mostly separate from that of Great Britain so comparisons can be problematic.