Boris Johnson will face a vote of confidence in the Commons after dozens of MPs called his leadership into question.
Almost 30 Tory MPs have publicly urged the Prime Minister to resign amid the fallout from revelations about Downing Street parties held during lockdown.
However it is thought many more MPs privately want the Prime Minister to step down.
On Monday morning, chairman of the 1922 committee Sir Graham Brady confirmed the threshold to trigger a leadership poll had been reached.
A vote is set to take place later in the day between 6pm and 8pm, with rebels requiring 180 MPs in order to oust the Prime Minister.
However allies of Johnson made clear he is determined to fight to stay on.
Sir Graham added that the Prime Minister was informed that the threshold had been met on Sunday evening.
He said some colleagues had post-dated their letters until after the Platinum Jubilee celebrations.
He 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.”
He refused to confirm how many letters had been 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”.
Under Conservative Party rules, if 54 letters from MPs are sent to Sir Graham then a vote is called.
In a statement on Monday morning, Sir Graham Brady 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 6pm and 8pm today, Monday June 6 — 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.”
A No 10 spokeswoman 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.”
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