Theresa May has promised Tory MPs she will stand down if they back her EU withdrawal deal.
She told backbench Tories: “I am prepared to leave this job earlier than I intended in order to do what is right for our country and our party.”
The PM said she knew that Tory MPs did not want her to lead the next phase of Brexit negotiations “and I won’t stand in the way of that”.
She did not name a departure date at a packed meeting of the 1922 committee.
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But the BBC’s Laura Kuenssberg said a Tory leadership contest could be expected in May.
Downing Street said it would be a “different ball game” if the deal was not passed by Parliament.
It comes as MPs seize control of the Commons agenda to hold votes on alternatives to the deal.
Mrs May told the 300 or so Tory MPs at the meeting “we need to get the deal through and deliver Brexit”.
“I ask everyone in this room to back the deal so we can complete our historic duty – to deliver on the decision of the British people and leave the European Union with a smooth and orderly exit.”
The BBC’s Iain Watson said Boris Johnson – a likely contender in any leadership contest – was smiling broadly as he left the meeting.
Our correspondent said a very senior Conservative had said the PM was “as clear as she has ever been” that she will not be around for the next stage of Brexit but if the deal does not pass then “that’s a different matter”.
Tory MP Simon Hart said the mood in the 1922 meeting was “respectful” as the prime minister set out her plan.
He said: “She was passionate about getting the deal through, passionate about keeping the party together and passionate about keeping the government as the government, passionate about keeping Jeremy Corbyn out of Number 10.”
Despite the nature of the meeting, Mr Hart joked that “neither the Chief Whip nor the PM were crying”.
The PM has said she wants to bring the deal back to the Commons this week, after it was previously rejected twice, by large margins.
Speaker calls for changes
Commons Speaker John Bercow ruled last week that the government could not return for a third attempt, unless there had been “substantial” changes to the proposals.
And he warned ministers earlier that they should “not seek to circumvent my ruling” by introducing procedures that could reverse his judgement.
But a Downing Street spokesman said there had been a “significant development” at the summit in Brussels last week, after Mrs May agreed “extra reassurances” over the Irish backstop with the EU, and the date of exit had changed.
Many Tory Brexiteers are looking to the Democratic Unionist Party, who have led opposition to the PM’s deal, before deciding whether to get behind it.
Leading Tory Brexiteer Jacob Rees-Mogg said there was an “element of sadness” about the prime minister’s announcement “even though it’s something I’ve wanted”.
And speaking to journalists after the 1922 committee he said he would vote for the government’s Brexit deal if the DUP abstained.
Asked what would happen if the government’s deal failed to get through, he said: “Then she would have every right to carry on.”
He refused to speculate on who would now stand as Conservative leader – but asked what he thought of Boris Johnson he said: “I think Mr Johnson is a formidably able man and I backed him in 2016.”
Earlier, Mrs May moved to prevent possible ministerial resignations by allowing Conservative MPs a free vote when MPs pass judgment later on different Brexit plans, in the so-called indicative votes.
But the prime minister herself, along with her Cabinet ministers, will abstain in the votes, Conservative whips have indicated.
Brexit: Theresa May vows to stand down if deal is passed}