Enough Tory MPs have requested a vote of no confidence in Theresa May to trigger a contest, the chairman of the backbench 1922 Committee has announced.
Sir Graham Brady said the threshold of 48 letters – 15% of the parliamentary party – needed to trigger a vote has been reached and a ballot will be held between 6pm and 8pm on Wednesday evening in the House of Commons.
"The votes will be counted immediately afterwards and an announcement will be made as soon as possible in the evening," he said.
Mrs May is expected to make a statement responding to the development outside the door of 10 Downing Street on Wednesday morning, a senior source told the Press Association.
The announcement followed reports of a wave of new letters amid anger at the way Mrs May dramatically put on hold the crunch Commons vote on her Brexit deal after admitting she was heading for a heavy defeat.
Earlier, unconfirmed reports suggested Sir Graham had asked to meet Mrs May after Prime Minister's Questions on Wednesday afternoon.
Former cabinet minister Owen Paterson was the latest MP to declare he had submitted a letter to the chairman of the backbench 1922 Committee.
Speculation that a challenge could be imminent was fuelled after chief whip Julian Smith and Conservative Party chairman Brandon Lewis were seen leaving No 10 following late-night consultations on Tuesday.
In a joint statement, the chairman of the European Research Group of eurosceptic Tory backbenchers Jacob Rees-Mogg and his deputy Steve Baker said: "Theresa May's plan would bring down the Government if carried forward. But our party will rightly not tolerate it.
"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."
In his letter, published in The Daily Telegraph, Mr Paterson said Mrs May's conduct of the Brexit negotiations had "eroded trust in the Government, to the point where I and many others can no longer take the Prime Minister at her word".
The former Northern Ireland secretary and prominent Brexiteer said she had become a "blockage" to an agreement which Parliament and the country could support.
"She has repeatedly said 'no deal is better than a bad deal', but it is clear her objective was to secure a deal at any cost," he wrote.