Sir Graham Brady, the chairman of the 1922 backbench committee, released a statement saying the threshold of 48 signatures – 15 per cent of the total number of Tory MPs – “has been exceeded”.
The announcement came after Brexiteer Tories, who had held back from submitting letters calling for a confidence vote, broke cover in anger at the shelving of the vote on the Brexit deal.
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The contest will be staged between 6pm and 8pm this evening, when all 315 Conservative MPs will vote on whether they believe Ms May should continue.
The prime minister requires 158 supporters to survive – after which there cannot be another contest for one year – although rejection by more than 100 Tory MPs could damage her fatally.
If Ms May is forced out, it could see Brexit delayed if the EU accepts the Article 50 exit period must be extended beyond next March because of the political turmoil.
A new Tory leader – and prime minister – would not be in place until the middle of January at the earliest, the deadline for the Commons to vote on the current deal.
Ms May was poised to make a statement in Downing Street this morning, at which she was expected to confirm she would fight to keep her job.
In a joint statement, the leadership of the hard Brexit-supporting European Research Group of Tory MPs said Ms May “must go”.
“Theresa May’s plan would bring down the Government if carried forward. But our party will rightly not tolerate it,” said Jacob Rees-Mogg and his deputy Steve Baker.
“Conservatives must now answer whether they wish to draw ever closer to an election under Mrs May’s leadership. In the national interest, she must go.”
Cabinet big hitters including Jeremy Hunt, the foreign secretary, Sajid Javid, the home secretary, and Amber Rudd, the work and pensions secretary, immediately threw their weight behind their leader.
Mr Javid said: “The last thing our country needs right now is a Conservative Party leadership election. Will be seen as self-indulgent and wrong.
“PM has my full support and is best person to ensure we leave EU on 29 March.”
Confirming the contest this evening, Sir Graham said: “The votes will be counted immediately afterwards and an announcement will be made as soon as possible in the evening.”
The Independent has launched its #FinalSay campaign to demand that voters are given a voice on the final Brexit deal.