Former Bury South Tory MP Christian Wakeford told Boris Johnson 'both you and the Conservative Party as a whole have shown themselves incapable of offering the leadership and government this country deserves' as he defected to join Labour.
The contents of Mr Wakeford's resignation letter was revealed as the news was confirmed before Prime Minister's Questions on Wednesday.
He said the country needed a government that 'upholds the highest standards of integrity and probity'.
But the Red Wall MP told Mr Johnson, who is battling for his political career amid the 'partygate' investigation, that 'both you and the Conservative Party as a whole have shown themselves incapable of offering the leadership and government this country deserves'.
Mr Wakeford won Bury South, which had elected a Labour MP at every election since 1997, in 2019 - with a majority of just 402, replacing Labour's Ivan Lewis.
He announced his decision in the Bury Times and sent a letter to Mr Johnson explaining why he had lost patience with his leadership.
Mr Wakeford said: "I care passionately about the people of Bury South and I have concluded that the policies of the Conservative government that you lead are doing nothing to help the people of my constituency and indeed are only making the struggles they face on a daily basis worse.
"Britain needs a government focused on tackling the cost of living crisis and providing a path out of the pandemic that protects living standards and defends the security of all.
"It needs a government that upholds the highest standards of integrity and probity in public life and sadly both you and the Conservative Party as a whole have shown themselves incapable of offering the leadership and government this country deserves.
"From today I will be sitting as the Labour MP for Bury South because I have reached the conclusion that the best interests of my constituents are served by the programme put forward by Keir Starmer and his party."
Mr Wakford said he had 'wrestled with his conscious for months'.
He said: "Being elected as MP for Bury South was the proudest day of my life.
"I care passionately about the area and will always be grateful to those who have supported me.
"Today, however, I am in no doubt that they will be better served by my joining a party that genuinely has their interests at heart."
He said the Conservative government has 'shown itself consistently out of touch with the hard working people of Bury South and the country as a whole'.
Changes, he concluded, 'can only be delivered by a Labour government'.
Mr Wakeford had been one of seven Tory MPs to publicly call for Mr Johnson to quit.
Labour leader Sir Keir Starmer said: "I would like to welcome Christian Wakeford to the Labour Party. He has always put the people of Bury South first.”
He added: "People across Britain face a cost-of-living crisis but this incompetent Tory government is asleep at the wheel, distracted by a chaos of its own making.
"Meanwhile families, businesses and pensioners are suffering from the Conservative failure to tackle rising food, fuel and energy prices."
