Tory MP Christian Wakeford has defected to the Labour Party and called on Boris Johnson to quit as prime ministerMr Wakeford took the Bury South seat from Labour at the 2019 general election.
He was among the MPs believed to have written a letter of no confidence in Mr Johnson, following revelations about lockdown parties in Number 10.
His defection came as Mr Johnson faced Labour leader Sir Keir Starmer at Prime Minister's Questions.
In a letter to Mr Johnson, Mr Wakeford told the prime minister that "you and the Conservative Party as a whole have shown themselves incapable of offering the leadership and government this country deserves".
He was cheered by Labour MPs as he crossed the floor of the Commons chamber to join his new colleagues.
In his letter, Mr Wakeford said Labour was "ready to provide an alternative government that this country can be proud of, and not embarrassed by".
He said the changes his constituents needed could "only be delivered by a Labour government with Keir Starmer".
Offering Mr Wakeford a "warm welcome" to his party at Prime Minister's Questions, Sir Keir said: "As Christian said, the policies of the Conservative government are doing nothing to help the people of Bury South and indeed are only making the struggles they face on a daily basis worse."
The left-wing Momentum campaign group said Mr Wakeford "should be nowhere near the Labour Party".
"Wakeford should be booted out of Labour and a by-election called immediately," a spokesman added. "Labour must back a candidate that will stand up for the people of Bury South and against this Tory government."
Mr Wakeford is one of the Red Wall Conservatives, who won traditional Labour seats in the north of England at the 2019 election.
The 37-year-old former insurance broker and Lancashire county councillor took Bury South by just 402 votes from Labour candidate Lucy Burke.