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.
Scope of these records 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 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:
Frequent interventions and withdrawals of parties in different seats. Frequent coalitions between parties, splits within parties and floor-crossing by members. Uncontested elections and truces between parties, in particular during both World Wars. Generally more significant competition from independent candidates and minor parties. Multi-member seats and university seats (abolished 1950). Higher frequency of by-elections, partly due to the practice of often uncontested ministerial by-elections which ended in 1926. Generally higher turnouts, although several wartime elections exhibited the lowest recorded turnouts. Generally higher variation in size of constituency electorates.
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.
Glossary For comparison purposes the following definitions have been adopted.
Gain: victory by a party that was not victorious at the immediate previous election Loss: defeat of a party that was victorious at the immediate previous election Hold: victory by a party that was victorious at the immediate previous election Win: victory by a party. ambiguous term that could mean either a gain or a hold Incumbent: the party that held the seat at the immediately previous election, irrespective of any intervening change of candidate or candidate's change of party Third party: In England, since 1922, the "third party" has been the Liberal Party and its successor, the Liberal Democrats. Additionally, in Scotland and Wales the Scottish National Party (SNP) and Plaid Cymru are also considered to be third parties. Prior to 1922, the third party was the Labour Party. Minor party: parties smaller than the third party Uncontested: an election where only one candidate is put forward. No votes are actually cast and the candidate is by definition the victor.
Numerical records For more information about what is meant by the term "swing", see Swing (United Kingdom)
Largest swings in percentage share of votes
ElectionSwingFromTo1983 Bermondsey by-election44.22014 Clacton by-election44.11973 Lincoln by-election43.02024 Rochdale by-election41.81967 Hamilton by-election37.92012 Bradford West by-election36.61993 Christchurch by-election35.42021 North Shropshire by-election34.21988 Glasgow Govan by-election33.11972 Sutton and Cheam by-election32.61979 Liverpool Edge Hill by-election30.22022 Tiverton and Honiton by-election29.91994 Dudley West by-election29.22023 Somerton and Frome by-election29.02003 Brent East by-election28.92024 Wellingborough by-election28.51993 Newbury by-election28.42014 Rochester and Strood by-election28.32004 Birmingham Hodge Hill by-election26.72026 Gorton and Denton by-election26.41962 Orpington by-election26.32024 Blackpool South by-election26.31973 Ripon by-election25.32021 Chesham and Amersham by-election25.11991 Ribble Valley by-election24.71981 Croydon North West by-election24.22023 Tamworth by-election23.92023 Selby and Ainsty by-election23.71994 Dagenham by-election23.11999 Hamilton South by-election22.61976 Walsall North by-election22.52008 Glasgow East by-election22.51996 South East Staffordshire by-election22.11994 Barking by-election22.02016 Richmond Park by-election21.72004 Leicester South by-election21.51990 Mid Staffordshire by-election21.31968 Dudley by-election21.21977 Ashfield by-election20.91976 Newcastle-upon-Tyne Central by-election20.71999 Leeds Central by-election20.52023 Mid Bedfordshire by-election20.52023 Rutherglen and Hamilton West by-election20.41990 Eastbourne by-election20.0
Largest swings to an incumbent governing party It is rare to see any swing towards the governing party in by-elections. However, there are some examples of it happening.
ElectionSwingFromTo2021 Hartlepool by-election16.01945 Bournemouth by-election10.31982 Mitcham and Morden by-election10.21878 Worcester by-election9.92017 Copeland by-election6.71978 Hamilton by-election4.52026 Makerfield by-election3.51945 Smethwick by-election2.91997 Beckenham by-election2.61945 Edinburgh East by-election2.01978 Berwick and East Lothian by-election0.8 *BOLD indicates winning party
Largest fall in percentage share of vote A party's share of the vote at a general election is not always matched at subsequent by-elections, but given the five-year maximum term of a Parliament, reductions of 20% or more are unusual. Those of 25% or more are listed below:
ElectionFall: %PartyResult1948 Glasgow Camlachie by-election51.3 gain2024 Rochdale by-election43.9 gain2024 Wellingborough by-election37.6 gain1983 Bermondsey by-election37.5 gain1969 Birmingham Ladywood by-election33.4 gain1993 Christchurch by-election32.5 gain2024 Blackpool South by-election32.2 gain1946 Glasgow Bridgeton by-election32.1 hold1958 Rochdale by-election31.7 gain2021 North Shropshire by-election31.1 gain1994 Dudley West by-election30.2 gain1995 North Down by-election29.9 gain from Popular Unionist1967 Hamilton by-election29.7 gain2004 Birmingham Hodge Hill by-election29.6 hold1961 Paisley by-election29.5 hold2003 Brent East by-election29.4 gain1993 Newbury by-election29.0 gain1968 Caerphilly by-election28.7 hold1999 Hamilton South by-election28.7 hold2023 Mid Bedfordshire by-election28.6 gain2014 Clacton by-election28.4 gain1962 West Lothian by-election28.3 hold1979 Liverpool Edge Hill by-election28.1 gain1958 Torrington by-election27.7 gain1968 Oldham West by-election27.6 gain2009 Norwich North by-election26.7 gain1933 Fulham East by-election26.6 gain1972 Sutton and Cheam by-election26.2 gain2023 Selby and Ainsty by-election26.0 gain2023 Tamworth by-election25.7 gain1948 Glasgow Gorbals by-election25.5 hold2019 Peterborough by-election25.5 hold2026 Gorton and Denton by-election25.4 gain1962 West Derbyshire by-election25.2 hold2004 Leicester South by-election25.2 gain
In the 1934 Merthyr by-election the Independent Labour Party share dropped from 69.4% in the 1931 general election to 9.8% (a record 59.6% loss) losing the seat to the Labour Party. However, the 1931 election had no Labour Party candidate, and the MP, R. C. Wallhead, had previously been elected as a Labour candidate in prior elections, when the ILP was affiliated to Labour. Prior to his death, Wallhead joined the Labour Party, so this result could be classed as a Labour hold.
The 1919 East Antrim by-election saw the Irish Unionist party face its first Unionist opposition in the seat since 1906 (in the 1918 general election the heavily unionist area gave the Irish Unionist 94.6% of the vote in a contest with a Sinn Féin candidate). An Independent Unionist candidate won the seat, with the Irish Unionist share dropping by 52.8%
Worst results for other parties:
ElectionFall: %PartyResult2021 Hartlepool by-election24.6 gain from Labour1982 Belfast South by-election22.4 hold2014 Heywood and Middleton by-election17.6 hold2023 Rutherglen and Hamilton West by-election16.6 gain2009 Glasgow North East by-election14.0 gain from Speaker2017 Copeland by-election9.0 gain from Labour1986 Newry and Armagh by-election7.7 gain from Ulster Unionist1963 Swansea East by-election5.3 hold
Largest increase in percentage share of vote
ElectionIncrease in SharePartyResult1986 East Londonderry by-election56.0 hold2012 Bradford West by-election52.8 gain1983 Bermondsey by-election50.9 gain1986 South Antrim by-election48.4 hold1986 East Antrim by-election47.5 hold1986 North Antrim by-election43.2 hold2016 Batley and Spen by-election42.6 hold1972 Sutton and Cheam by-election39.0 gain1993 Christchurch by-election38.6 gain1988 Glasgow Govan by-election38.4 gain2022 Tiverton and Honiton by-election38.1 gain2021 North Shropshire by-election37.1 gain1979 Liverpool Edge Hill by-election36.8 gain2014 Heywood and Middleton by-election36.1 hold1986 Belfast East by-election35.7 hold1986 Belfast North by-election35.3 hold1973 Glasgow Govan by-election31.6 gain1986 Lagan Valley by-election31.5 hold2021 Chesham and Amersham by-election30.4 gain1973 Ripon by-election30.4 gain2016 Richmond Park by-election30.3 gain1981 Croydon North West by-election29.5 gain1968 Caerphilly by-election29.3 hold2003 Brent East by-election28.5 gain1994 Dudley West by-election28.0 gain1987 Greenwich by-election27.9 gain2026 Gorton and Denton by-election27.4 gain1972 Merthyr Tydfil by-election27.4 gain1966 Carmarthen by-election27.4 gain1991 Ribble Valley by-election27.1 gain1994 Monklands East by-election26.9 hold1931 Gateshead by-election26.9 hold2022 Southend West by-election26.9 hold2004 Birmingham Hodge Hill by-election26.1 hold2008 Glasgow East by-election26.1 gain2026 Aberdeen South by-election25.1 gain
Largest winning share of the vote Winning shares of the vote above 90%, since 1918:
CandidatePartyElectionVotes% ShareErnest Gates1940 Middleton and Prestwich by-election32,03698.7Ian Paisley1986 North Antrim by-election33,93797.4John Craik-Henderson1940 Leeds North East by-election23,88297.1Charles Key1940 Bow and Bromley by-election11,59495.8John Taylor1986 Strangford by-election32,62794.2Clifford Forsythe1986 South Antrim by-election30,08794.1William Ross1986 East Londonderry by-election30,92293.9Arthur Woodburn1939 Clackmannanshire and East Stirlingshire by-election15,64593.7Spencer Summers1940 Northampton by-election16,58793.4Harry Thorneycroft1942 Manchester Clayton by-election8,89293.3James Hollins1940 Silvertown by-election14,34392.8Francis Douglas1940 Battersea North by-election9,94792.6Henry Willink1940 Croydon North by-election14,16390.7James Henry Molyneaux1986 Lagan Valley by-election32,51490.7Thomas Galbraith1940 Glasgow Pollok by-election17,85088.1William Beveridge1944 Berwick-upon-Tweed by-election8,79287.4John Peto1941 Birmingham King's Norton by-election21,57386.9William Henry Guy1942 Poplar South by-election3,37586.2Tracy Brabin2016 Batley and Spen by-election17,50685.8Adam McKinlay1941 Dunbartonshire by-election21,90085.0-->
Largest numerical majority overturned Majorities over 9,000 votes overturned: