Tony Blair's most senior Cabinet colleagues plan to urge him to stand down within 18 months because he lost seats for Labour at last week's election.
Even normally loyal ministers want him to resign by the time the party holds its conference in September next year - two years earlier than Blair wishes.
Left-wing MPs will demand this week that he drops controversial plans for identity cards, and threaten to mount a leadership challenge against him later in the year.
But insiders believe his own Cabinet poses a much bigger threat, saying it will turn against him if he tries to hang on for longer than 18 months.
Yesterday, ministers publicly rallied around Blair, condemning as "self-indulgent" backbenchers' calls for him to quit sooner rather than later.
But Cabinet sources said privately the Prime Minister will not be able to complete anything like the full term he intends to serve before leaving Downing St ahead of the next general election.
His most senior ministers will warn him he cannot survive for another three or four years, because the election result showed the voters no longer trust him in the aftermath of the Iraq war.
Some want him to announce his departure after the referendum on Europe planned for May or June next year - whatever the result.
One senior minister told the Independent: "He can't tough it out after that election result. We can't have another four years of this. I would give him 18 months. Tony should fight the European referendum and then go. That is beginning to look like a very attractive timetable."
Blair's third term has started badly. There is anger in the Cabinet about the way he handled Friday's reshuffle, when he was forced to drop three of his proposals: to remove some responsibilities from John Prescott and Charles Clarke and to move Ruth Kelly from the Department for Education and Skills.
Over the weekend, more than a dozen Labour MPs called in the media for Blair's early departure, warning he had become a liability.
One minister admitted: "The backbenchers are all saying they won their seats despite Tony, not because of him."
But Blair aides insisted yesterday there was no change to his plans to carry on until at least the autumn of 2008, despite seeing Labour's majority cut by almost 100.
Today he will outline his plans for a new round of public service reforms, and tomorrow he will address the Labour caucus. Although he will promise to learn lessons from the election, he will insist he has a majority and a mandate to bring in the policies in the manifesto.
Loyalist backbenchers will rally behind him, saying the MPs who spoke out against him at the weekend were not representative of the Parliamentary Labour Party.
Gordon Brown, the overwhelming front-runner to succeed the Prime Minister, spoke to Blair yesterday and both men agreed the resignation calls were "unhelpful" and "a distraction".
The Chancellor pledged to maintain the backing he gave Blair during the election. Backbench allies of Brown say he wants "unity in transition". But they believe he will not want to wait until 2008 to take over.
The Prime Minister is expected to announce a reshuffle of junior and middle-ranking ministers. In an attempt to draw a line under the Iraq conflict, he will recall to a senior post John Denham, who resigned as a Home Office minister in protest at the decision to go to war.
Frank Dobson, the former Health Secretary, and Robin Cook, the former Foreign Secretary, said Blair should consider quitting before the council elections in a year to prevent Labour suffering further losses.
Dobson said: "He was an enormous liability in this general election. If he had not been leader I doubt whether we would have lost a seat. We would probably have gained some."
Blair loyalist David Blunkett, who has been appointed Work and Pensions Secretary, said "self-indulgent" MPs calling for Blair to go were trying to "snatch defeat from the jaws of victory".
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Ministers give Blair 18-month deadline to go
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