Cabinet ministers have assured rebel Labour MPs that they will try to force Gordon Brown out of Downing Street in November if he fails to revive the party's prospects by then.
There have been private contacts between rebel leaders and some cabinet ministers over the weekend. They have signalled that they will move against Mr Brown if he fails three big tests – a successful relaunch, a good Labour conference in Manchester next week and avoiding defeat in a difficult by-election in Glenrothes, Fife, expected in late October or early November.
Ministers insist they are not setting a deadline and want the whole party to rally behind Mr Brown as they give him a final opportunity to save his premiership.
One said last night: "Now is not the right moment. But we will take stock after the series of hurdles that lie ahead. We have got to resolve this one way or the other by Christmas."
A dozen Labour backbenchers have openly challenged Mr Brown's authority by calling for a leadership election. They want nomination forms to be sent to all Labour MPs before next week's conference.
Although they have little chance of securing the 70 names for a rival candidate needed to trigger a contest, they hope that a nomination process would reveal how Mr Brown has lost support since last year, when only seven of Labour's 350 MPs did not nominate him to succeed Tony Blair. Rebels claim as many as 100 backbenchers might refuse to back him now. More backbenchers are expected to join the call for a leadership contest in the coming days.
The Independent has learnt that there will be demands for an emergency meeting of the Parliamentary Labour Party this week to discuss criticism of its chairman, Tony Lloyd.
He is accused by rebels of misrepresenting the level of dissent in the PLP. He described the rebels yesterday as "a small group" who were "seeking to undermine the Government".
Brown critics admit their action was designed to provoke a cabinet mutiny in which senior figures would ask Mr Brown to quit. Fiona Mactaggart, a former Home Office minister, urged senior ministers to show "courage" and expressed confidence that some supported the rebels privately.
Private doubts among ministers surfaced when John Hutton, the Business Secretary, took a more emollient line towards the rebels than Downing Street had wanted. While backing Mr Brown, he told the BBC's Andrew Marr Show: "I'm not gong to criticise any of my colleagues who want Labour to do better and neither am I going to criticise those who say, for example, that we do need to set out a stronger vision of what we are doing."
Brown allies said an attempted coup had "gone off at half-cock". Tomorrow, the party's national executive committee is expected to quash the demands for nomination forms.
The Brown camp welcomed declarations of support for the Prime Minister by cabinet colleagues, including the Foreign Secretary, David Miliband. It was relaxed about Mr Hutton's remarks, saying the Prime Minister had ordered that the rebels should not be vilified.
Brown allies vented their anger at the rebels. Nick Brown, Labour's deputy chief whip, said: "If they have not got the support [for a challenge] and are just trying to drum it up, then they are doing a substantial amount of damage to the Labour Party and they should pack it in."