Boris Johnson faces a crunch vote of no confidence tonight as mutinous Tories bid to oust him from Downing Street.
Conservative 1922 Committee chairman Sir Graham Brady confirmed that the 54-letter threshold to trigger a leadership challenge had been reached, teeing up a secret ballot latter.
Mr Johnson now faces a desperate fight for his political survival, barely two years after winning an 80-seat landslide in the 2019 general election.
The secret ballot of Tory MPs is expected to be held tonight between 6pm and 8pm, with a result to follow soon after.
In a statement, Sir Graham said: "The threshold of 15% of the parliamentary party seeking a vote of confidence in the leader of the Conservative Party has been exceeded.
"In accordance with the rules, a ballot will be held between 1800 and 2000 TODAY MONDAY 6th JUNE - details to be confirmed.
"The votes will be counted immediately afterwards. An announcement will be made at a time to be advised. Arrangements for the announcement will be released later today."
While only 54 letters are needed to trigger a vote, at least half of Tory MPs - about 180 - would need to vote against Mr Johnson to oust him from office.
If he survives the vote, Mr Johnson will be immune from a leadership challenge for another year.
This is what happened to Theresa May in 2018 but she resigned months later after struggling to get her Brexit deal through Parliament.
However, Sir Graham confirmed the rules could be changed if the 1922 wants to. Members have previously discussed shortening the grace period to hold another vote after six months.
Downing Street suggested the PM would come out fighting - and said it would be a chance for MPs to "draw a line" under the leadership question.
A No 10 spokesperson said: "Tonight is a chance to end months of speculation and allow the government to draw a line and move on, delivering on the people’s priorities.
"The PM welcomes the opportunity to make his case to MPs and will remind them that when they’re united and focused on the issues that matter to voters there is no more formidable political force."
Mr Johnson wrote a begging letter to MPS urging him to back him - and tried to blame the criticism on the "media's obsession".
He said he had learned from the criticism and he accepted that some of it was fair.
But the numbers of his critics began to swell.
In an explosive move, Jeremy Hunt, who stood against Boris Johnson in the 2019 contest, said that he would vote against the PM.
The ex-Foreign Secretary said: "Having been trusted with power, Conservative MPs know in our hearts we are not giving the British people the leadership they deserve.
"We are not offering the integrity, competence and vision necessary to unleash the enormous potential of our country.
"And because we are no longer trusted by the electorate, who know this too, we are set to lose the next general election.
"Anyone who believes our country is stronger, fairer and more prosperous when led by Conservatives should reflect that the consequence of not changing will be to hand the country to others who do not share those values.
"Today's decision is change or lose. I will be voting for change.
Keir Starmer said Conservative MPs must "show some leadership" and vote against Boris Johnson.
Speaking to LBC Radio, the Labour leader said: "He has lost, I think, the trust of the country - that is pretty clear from the evidence that I have seen."
Sir Keir said the public mood has changed with a "general sense that this man doesn't really tell the truth".
Mr Johnson was told he was facing a vote on Sunday night, which suggests Downing Street was keen to get the vote over and done with the prevent further plotting.
Sir Graham told reporters in Westminster: "I notified the Prime Minister yesterday that the threshold had been reached.
"We agreed the timetable for the confidence vote to take place and he shared my view - which is also in line with the rules that we have in place - that that vote should happen as soon as could reasonably take place and that would be today."
The backbench chief refused to say how many letters he had received or when the threshold had been passed but said "it is slightly complicated because some colleagues had asked specifically that it should not be until the end of the Jubilee celebrations".
Ex-minister Jesse Norman broke cover on Monday morning with a blistering letter branding Mr Johnson "grotesque" for his response to Partygate.
He said Mr Johnson “absolutely must step down now or be removed” due to a “wider set of concerns” which aren’t just about Partygate.
The ex-minister told BBC Radio 4’s Today programme Rwanda asylum removals and Northern Ireland Brexit policies are both “illegal, or potentially illegal”.
And he said the PM is pursuing policies to “create dividing lines and culture war and political disagreement, just at a time when we need unifying, inclusive, energetic positive leadership”.
The MP also raised concerns over the centralisation of power in No10 that is making Boris Johnson into more of a “President” in a “serious mistake”.
It comes after Mr Johnson risked a no confidence vote earlier this year when he was accused of lying over Partygate.
He claimed “nobody told me” a ‘BYOB’ No10 bash in May 2020 was against the rules. 'Red Wall' MPs elected in 2019 led a furious revolt dubbed the 'pork pie plot'. It fizzled when one, Christian Wakeford, announced a shock defection to Labour.
Anger among Tory MPs resurged when the Sue Gray report into lockdown parties found "failures of leadership" in No10, and police announced they were investigating 12 gatherings in Downing Street and Whitehall.
It then became worse when the Prime Minister tried to defend himself by smearing Keir Starmer, repeating a far-right smear that he failed to prosecute Jimmy Savile. Five aides quit No10 in the aftermath including Munira Mirza, one of his longest-serving advisors who left with an excoriating attack on the Prime Minister.
Mr Johnson managed to steady the ship with staffing changes in No10, and that - together with a focus on the Russia-Ukraine war - meant some Tory MPs withdrew their letters.
But calls for him to quit resurged after Sue Gray's final report showed him toasting with fizz on 13 November 2020 - the day he solemnly promised no party happened at No10.
More than two dozen MPs publicly called for their leader to leave - but letters to the 1922 Committee are private, meaning more were urging their boss's removal without speaking up to the press.
A full list of the Tory MPs who have publicly called for the Prime Minister's resignation can be seen here.
