MPs (and councillors and regional representatives) can change their party allegiances at any time with no formal conditions or penalties. The debatable constitutional principle here is that we vote for the person, not the party label - more on this later. Defecting MPs are often described as ‘crossing the floor of the House’ because it usually entails going from the governing side of the chamber to the opposition side or - less commonly - vice versa.

One MP who did this literally and visibly was right-wing Conservative Natalie Elphicke who rather inexplicably defected to Labour in May 2024, walking across the floor just ahead of Rishi Sunak’s PMQs that day. She stood down at the 2024 general election and so did not put her move to the democratic test. Perhaps the most famous defector in UK party politics was Winston Churchill, who left the Conservative Party for the Liberals in 1904 and switched back twenty years later. As he famously said in 1924, ‘Anyone can rat, but it takes a certain amount of ingenuity to re-rat.’

The UK party system is currently in almost unprecedented flux, with both ‘main’ parties on the slide and smaller parties on the rise. As both cause and consequence we are therefore seeing an unusually high number of defections across the parties.

Who’s going where - and why?

The Conservatives have fallen most dramatically in the polls, and in September 2025 shadow cabinet minister Danny Kruger became the first sitting Conservative MP to defect to Reform UK. (When Lee Anderson MP joined Reform in March 2024, he had already lost the Tory whip for refusing to apologise after saying ‘Islamists’ had ‘got control of London’ and its mayor Sadiq Khan.) To date, 17 more former Conservative MPs who had already lost their seats have made the same journey, including Nadine Dorries, former party chair Jake Berry,Marco Longhi, David Jones, and Andrea Jenkyns (now mayor of Lincolnshire, a first for Reform). Some of them have said in as many words, ’The Tory party is dead’. Dorries - perhaps the most devoted fan of former PM Boris Johnson - said ‘The Conservative Party cannot win the next election. It removes election-winning prime ministers and replaces them with duds.’

For some, it may be a question of ideological differences; many say, ‘I didn’t leave the party, the party left me’. Jones said that many defectors believe the Conservatives have morphed into ‘a blue-rinsed version of the Liberal Democrats’. And Jenkyns said that ‘none of the One Nations who ruined the Conservatives’ would be allowed into Reform. For others, it may be a reluctance to leave the heady world of politics; or, as some critics have said, ‘jumping ship for personal and political gain’. Opponents of both parties like to caution Reform voters who have abandoned the Conservatives, ‘Vote Reform and you get failed Tories’.

A couple of former Conservative MPs and one MSP have defected to the Liberal Democrats because of the former government’s handling of Brexit and the feeling that the Tories have moved too far to the right. Former Conservative MSP Jamie Greene said the party had become ‘Trump-esque in both style and substance’. The Lib Dems won 61 seats from the Conservatives at the last general election, and they now hope to attract more Conservative MPs, especially those who oppose Kemi Badenoch’s unenthusiastic stance on net zero and her desire for the UK to leave the European Convention on Human Rights.

So far, just one (very) former Labour MP has joined Reform: Simon Danczuk was suspended by Labour in 2015 for sending messages to a 17-year-old girl and he lost his seat in the 2017 election. He joined Reform in March 2024, stood for them in the 2024 Rochdale by-election, and lost. Also, of course, former Labour and now independent MPs Jeremy Corbyn and Zarah Sultana are in the process of creating a new group provisionally called Your Party. In October, three former Green Party councillors in Glasgow joined Your Party, three Labour councillors joined the Greens and one has joined Reform, as have dozens of Conservative councillors. It’s all go.

Is it democratic?

Defections invariably prompt opponents to demand that a by-election is held to legitimise the politician’s new party identity - and, almost always, the demand is rejected. There is no legal requirement for defectors to submit to a new vote by electors. Whether this is seen as a moral obligation depends on whether electors vote for the party label or the person. UK constitutional practice follows the ‘Burkean model’ of representation, that we are voting for the person and not the party. As Kruger put it, ‘You vote for an MP and then they are trusted - left alone - to use their own judgement in the interests of their constituents’. A by-election is therefore deemed unnecessary. However, many - probably most - voters usually vote on the basis of a party label (indeed, only about one-third of voters can name their MP, never mind their councillors). This is sometimes called the ‘party mandate model’ - that MPs are bound to the manifesto of party policies which they defended at the last election. In a recent YouGov poll (prompted by Kruger’s defection), two-thirds of respondents said that MPs who defect should be required to resign and fight a by-election. There have been a couple of attempts via private members’ bills to compel by-elections, both unsuccessful. Voters are often left feeling betrayed and no longer represented. More often than not, they punish defectors at the next election by voting them out of office.

There have been a (very) few honourable exceptions where defectors have submitted to a by-election, notably in 2014 when Douglas Carswell and Mark Reckless defected from the Conservatives to UKIP, fought by-elections and won. However, a May 2025 Ipsos poll found that 40% of voters think it is unacceptable for an MP to defect to another party at all.