Is there a by-election when an MP changes party?

Are by-elections automatic when an MP defects?

There are no rules requiring a by election if an MP leaves one political party for another, although sometimes the MP chooses to resign and fight a by-election. The convention that the MP changing parties does not resign to fight a by-election accords with the arguments of Edmund Burke, an MP and political theorist in the late 18th century. Burke was himself a rebel in a number of policy areas and considered that an MP was a representative rather than a delegate.

Historically, the Commons has acted on the principle that all MPs are individually elected, and voters put a “cross against the name of a candidate”. While decisions on candidates may be affected by their party labels, MPs are free to develop their own arguments once elected, until it is time to face the voters in the next general election.

Are there any sanctions or other forms of discouragement for crossing the floor?

There are no formal sanctions but discouragement comes in many forms, including the reactions of former party colleagues and the electorate.

Although MPs who change party allegiance during the course of a Parliament rarely stand for their new party in the same constituency at the following election, crossing the floor does not mean political death. There are examples of MPs being found safe seats by their new party and even taking their place on the front bench.

Is there any pressure to take action against MPs who defect?

Occasionally there are calls for MPs to resign or for by-elections to be triggered if they change parties. For example, in 2010, it was suggested that “Members should be required to cause a by-election if they defect to a different party from the one on whose manifesto they were elected”.  However, the government said that such a change would be “a major constitutional reform of the role of Members of Parliament and their independence” and that it had “no plans” to make such a reform.¹

Since then, there have been private members’ bills calling for constituents to be allowed a say in the matter and an e-petition calling for automatic by-elections following a defection.

Private members’ bills

On 2 September 2020, Anthony Mangnall MP used the Ten-Minute Rule procedure to introduce the Recall of MPs (Change of Allegiance) Bill. He said that his bill would add a new trigger to the Recall of MPs Act 2015: “any MP who voluntarily leaves the political party they represented upon their election to the House of Commons becomes subject to a recall petition”.² If the MP concerned was recalled, a by-election would be triggered. For more information on recall petitions, see the Library research briefing on Recall elections.

Anthony Mangnall was given leave to introduce his bill, after a vote in the House of Commons.³

In opposing the bill, Steve Baker MP commented that the bill “establishes the principle that we are here contingent on our membership of party”. He said that one of the problems of such a contention was that it could “compromise [an MP’s] ability to vote as our conscience dictated was best for our constituents and the nation”.⁴

On 23 November 2011, Chris Skidmore MP introduced the Members of Parliament (Change of Political Party Affiliation). He argued that it was “neither right nor fair that a constituent should live with that decision [an MP’s decision to change parties], often for many years, until a general election is called”.⁵

E-petition

E-petition 737600 calls for by-elections to be called automatically when MPs defect to another party:

When an MP decides they want to defect to another party a by-election should be automatically triggered to allow the constituents the opportunity have their democratic right to agree or not with their elected official.

Regardless of political views I believe you vote for both the individual candidates and their stance on issues and the more general direction and policies of the party they stand for. When an MP decides that they no longer wish to be a member of the party they stood for when you voted for them the electorate should have the opportunity to also change their mind by voting in a by-election.

E-petitions remain open for signatures for six months. This e-petition closed on 4 March 2026. It attracted 129,333 signatures.

The Petitions Committee considers whether all e-petitions that reach 100,000 signatures should be debated. At its meeting on 3 February 2026, the committee decided that the e-petition should be debated on 16 March 2026.

The UK Government responds to e-petitions that reach 10,000 signatures. The government’s response was published on the petition’s page on 9 February 2026. It said:

There are no plans to make changes to the current constitutional arrangements, whereby voters elect individual candidates, and not the political party they represent.

Who has crossed the floor?

True instances of crossing the floor – leaving the government for the Official Opposition or vice versa – are rare. In the last 20 years, just three MPs have moved from the Official Opposition to the governing party, and three have gone in the opposite direction.

Official Opposition to government

• Quentin Davies crossed the floor from the opposition Conservative Party to the governing Labour Party in June 2007. He was given a peerage in 2010.
• Robert Jackson left the Conservative Party for the Labour Party in January 2005. He left Parliament at the next election, in May 2005.
• Shaun Woodward, who was elected as the Conservative MP for Witney in 1997, crossed the floor in December 1999. At the 2001 general election, he was elected as the Labour MP for St Helens South.  He went on to serve as Northern Ireland Secretary from 2007 to 2010 in the Labour Cabinet.

Government to Official Opposition

In the 2019 Parliament, three Conservative MPs crossed the floor to join the Labour Party:

• Natalie Elphicke in May 2024. He left Parliament at the next election in July 2024.
• Dan Poulter in April 2024. He left Parliament at the next election in July 2024.
• Christian Wakeford in January 2022. He won Bury South, the seat he originally won as a Conservative in 2019, for Labour in 2024.

Previous examples of MPs who defected from one party to another include Winston Churchill and Reg Prentice who both held ministerial office in two different parties.

Churchill left the Conservative Party in May 1904, joining the Liberal Party and becoming a minister in 1905. He was a defeated Liberal candidate in the 1922 and 1923 general elections, and defeated as an independent in a 1924 by-election. In the 1924 general election he was elected as a Constitutionalist and became Chancellor of the Exchequer in the Conservative government following that election.

Reg Prentice was Labour’s Secretary of State for Education from 1974 to 1975 but following deselection by his constituency party, he left for the Conservative Party in 1977. From 1979 to 1981, he was a Minister of State of in Margaret Thatcher’s first Conservative Government.

When have MPs triggered by-elections?

in the 2010 Parliament, both Douglas Carswell and Mark Reckless stood down as MPs after leaving the Conservative Party for UKIP. Both won the subsequent by-elections.⁶ However, their decisions to stand down and contest the seats under for their new party were unusual.

Previous examples include Bruce Douglas-Mann and Dick Taverne. Bruce Douglas-Mann voluntarily triggered a by-election in 1982, in the Mitcham and Morden constituency, when he left Labour for the Social Democratic party. In 1973 Dick Taverne resigned from the Labour party, only to call a by-election and be re-elected under the banner of Democratic Labour.

Further reading

The Library has produced a briefing – Members’ Changes of Allegiance. It details moves by MPs from one party directly to another and instances of MPs losing or resigning a party whip.