Boris Johnson is to face a confidence vote later today that could see the end of his reign as Prime Minister.
Sir Graham Brady, chairman of the 1922 committee, said that at least 54 letters of no-confidence in the party leader have now been submitted.
The move follows months of pressure over the rule-breaking lockdown parties in Downing Street capped by the damning findings of the Sue Gray report.
Things are not looking good for the PM after crowds booed him in London on Friday, and MPs will have had the chance to gauge the mood in their constituencies over the bank holiday weekend.
Sir Graham said in a statement: "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 - 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 majority of Tory MPs will need to vote to remove the Prime Minister, meaning the rebels will need to get 180 votes in total.
Any Conservative leader who survives a confidence vote is safe from any further challenge for a year.
Sir Roger Gale, Conservative MP for North Thanet, told BBC Radio Four's Today programme the party is "spoilt for choice" when it comes to options for a new leader.
Asked if he knew who he would back to replace the Prime Minister, Sir Roger said: "There is a list of people, and you know the list as well as I do, who are likely to run.
"Any single one of those people, in my view, would make a better prime minister than the one that we've got at the moment, and as I said, I think we're spoilt for choice.
"There are some very safe pairs of hands there. I won't name names because the moment I do, it'll be assumed that I'm backing that person."
Defence Secretary and former Scottish Conservative MSP Ben Wallace is the favourite among Tory members, although Rishi Sunak, Liz Truss and Michael Gove are likely to be the frontrunners.
Meanwhile, Jesse Norman has become the 29th Tory MP to announce that he has submitted a no-confidence letter in Mr Johnson.
He said the PM had presided over "a culture of casual law-breaking" in No 10 and that his claim to be vindicated by the Sue Gray report was "grotesque".
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