What happens when an MP defects to another party?
After sitting Conservative MP Danny Kruger defected to Reform UK, Paste BN takes a look at the process and if there are any rules politicians have to follow when switching sides.
Wednesday 17 September 2025 12:20, UK
Danny Kruger has become the first sitting Conservative MP to defect to Reform UK.
The East Wiltshire politician joins 13 ex-Tory members who have joined Nigel Farage's party in recent months - but he is the only one who currently sits on the Tory frontbench.
Mr Kruger is far from the first MP to change sides between elections, but questions always remain about what it means for the constituents who they represent.
So, what exactly happens when an MP defects to another party?
According to the Institute for Government, MPs are allowed to change their party affiliation at any time.
The moment of defection itself often comes as a surprise, with negotiations happening behind closed doors, and the key piece of theatre occurs when a member "crosses the floor" - meaning they switch the part of the Commons' benches they sit on to join their new party.
Why isn't there automatic by-elections?
Despite the usual loud cries from an MPs former party that they should call a by-election to give voters a choice over which rosette their MP wears, there is no rule forcing them to.
The House of Commons Library puts the convention down to the arguments of a rebellious MP from the late 18th century, Edmund Burke, saying members are a representative of their constituency, rather than a delegate of any party.
"Historically, the Commons has acted on the principle that all members of the House of Commons are individually elected, and voters put a 'cross against the name of a candidate'," it says.
"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."
However, some choose to go to the public anyway, as was the case when Douglas Carswell and Mark Reckless headed from the Conservative Party to UKIP - the UK Independence Party - before the Brexit referendum.
There is no sanction for the move either - though MPs who have defected often speak of the cold shoulder they receive from their former colleagues in the corridors, or even from their new party, who can be untrusting of their conversion to their cause.
It is often the case that MPs who have changed party lose their seat at the next election when voters have their say, or choose to stand down when the poll comes.
If they do wish to stand again under their new banner, however, they have to go through the standard selection process carried out by local constituency parties, usually involving a vote for their preference, and be approved by the leadership before being named the electoral candidate.
How often does it happen?
Before Mr Kruger, former Conservative culture secretary Nadine Dorries announced she was defecting to Reform.
Then Maria Caulfield, who served as the MP for Lewes for nearly a decade, as well as a government minister, also announced she is defecting.
However, it remains an uncommon practice for MPs to defect.
From 1979-2024 there were only 202 cases of MPs changing allegiance, according to the Institute for Government - and over half of those being because they were suspended or kicked out of their existing party, rather than resigning to join a different side.
Until last year, it was even rarer for a member of the governing party to switch to the opposition.
In May 2024, in a dramatic move just moments before Prime Minister's Questions got underway, former MP for Dover, Natalie Elphicke, joined Labour as her surprised Conservative colleagues looked on.
She was the third Tory to do so in just over two years, following Christian Wakeford in 2022 over the partygate scandal and Dan Poulter over the Tories' handling of the NHS.
Before that, it hadn't happened since 1995, when Alan Howarth became the first ever Conservative to cross to Labour on the eve of his party conference, citing the "divisiveness" of policies under John Major's leadership.
Only three MPs in the past 25 years have gone the other way - from main opposition to government - all leaving the Conservatives to join Labour.
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Defecting to smaller parties happens more often, such as moves from both Conservative and Labour to Change UK during the Brexit years.
And the SNP's Lisa Cameron moved onto the Tory benches in 2023 over claims of mistreatment and bullying by her now former party colleagues.