Lee Anderson’s decision to join Reform UK has prompted calls for a by-election in his Ashfield seat, where he was elected in 2019 as a Conservative.

There is no requirement for MPs to stand down and trigger a by-election if they defect from their original party, as they are formally elected as an individual rather than a party representative.

There have been several attempts to change this, mainly through Bills introduced by backbench MPs, but none has so far successfully made it on to the statute book.

In the past, MPs have been reluctant to put themselves to the test of a by-election after switching their allegiance, especially as it is rare for defectors to retain their seats.

Since the last election, there have been five defections, none of which resulted in a by-election.

But there have been exceptions.

In 2014, Conservative MPs Douglas Carswell and Mark Reckless not only defected to Ukip but also resigned their seats to contest by-elections for their new party.

Douglas Carswell first day as Ukip MP
Former Tory Douglas Carswell decided to trigger a by-election after his defection to Ukip in 2014, providing the Eurosceptic party with a demonstration of its strength and its first directly elected MP (Andrew Matthews/PA)

The move was seen as a sign of the Eurosceptic party’s strength and confidence, and resulted in re-election for both Mr Carswell and Mr Reckless, adding to pressure on David Cameron to include the promise of a referendum on EU membership in the 2015 Tory manifesto.

Mr Carswell was also the last defector to retain his seat at the following general election, winning re-election as Ukip MP for Clacton in 2015.

Prior to that, no defector had kept their seat since 2005 when Sir Jeffrey Donaldson retained Lagan Valley at the general election after leaving the UUP for the DUP.

Announcing his own defection to Reform, Mr Anderson ruled out the possibility that he would trigger a by-election, citing the impending general election as a reason.

He said: “It would be pretty reckless for me to suggest a by-election when we could have a general election in May.”

But Mr Anderson has previously backed calls for MPs who switch parties to be required to stand down and fight a by-election.

Lee Anderson
Lee Anderson has defected to Reform UK (Lucy North/PA)

In September 2020, he voted for Tory MP Anthony Mangnall’s Recall of MPs (Change of Party Affiliation) Bill, which would have introduced such a requirement.

Reacting to Mr Anderson’s defection, Mr Mangnall tweeted: “Given that Lee Anderson backed and signed my recall motion, stating that if MPs cross the floor to another political party I hope he is calling a by-election.”

Fellow Tory MP Chris Clarkson retweeted his comment, adding: “Just like the last Conservative MP to defect did… Men of principle, all.”