David Miliband is preparing to throw his hat into the ring in a leadership contest to “save new Labour” after the party’s disastrous defeat in last week’s Crewe & Nantwich by-election.

The foreign secretary has confided to friends that he is prepared to stand for the leadership if a critical mass of backbenchers turn against Gordon Brown.

He is discussing a strategy to position himself for the top job without personally engineering the prime minister’s downfall.

It comes as senior Labour insiders claim that at least half the cabinet have privately concluded that Brown cannot win the next general election.

Among the ministers said to be ready to desert Brown are Alan Johnson, the health secretary, Ruth Kelly, the transport secretary, James Purnell, the work and pensions secretary, Jacqui Smith, the home secretary, John Hutton, the business secretary, Hazel Blears, the communities secretary, Jack Straw, the justice secretary, and Tessa Jowell, the Olympics minister.

In an alarming development for No 10, some insiders claim that Brown is also losing the support of Alistair Darling, the chancellor, regarded as one of his most loyal allies.

In the past 48 hours Miliband has held private talks with friends and advisers over the leadership crisis. Publicly he is denying any plan to stand for the party leadership and has decided not to make any move until a vacancy arises.

Following Labour’s catastrophic defeat in Crewe, he was one of the few cabinet ministers publicly to defend Brown, declaring that he was the “right leader” for the party. Insiders say he issued the statement only after a request from No 10 to back the prime minister.

Downing Street was this weekend pressing other ministers seen as a threat to Brown to take to the airwaves with similar declarations of loyalty.

Miliband’s supporters hope that a delegation of cabinet ministers will persuade Brown to stand down voluntarily in the interests of the party. They believe that Straw, Johnson, Geoff Hoon, the chief whip, and John Denham, the higher education secretary, could play a key role because of the respect that they command among backbenchers.

A source close to Miliband said: “David is not going to do anything until a vacancy arises, but he is ready to go for it. There will be no public display from him in the next few weeks but he and his supporters will be making it clear to backbenchers that there is an alternative to Gordon.”

Last spring Miliband came under intense pressure to challenge Brown for the party leadership when Tony Blair stood down, but concluded that he could not win.

Purnell is also understood to be considering running, while Straw is believed by many MPs to be positioning himself, despite publicly denying the move.

Backbenchers are still divided over whether Brown can lead the party to victory in the next general election, with many continuing to voice support for him. The prime minister and his allies blame the dramatic loss of support for the party on the economy and insist that voters will return to Labour when the credit crunch passes. This weekend they were taking heart from the fact that no senior Labour figure had been emboldened to criticise the prime minister publicly.

None of the three former Blairite ministers fiercely critical of Brown — Alan Milburn, Charles Clarke and Stephen Byers — has spoken out since the Crewe result. Sources close to the three, however, say their silence signals the seriousness of Brown’s situation because they are now confident that serving ministers will lead an attack.

Last night it was reported that some ministers want Brown to appoint a deputy prime minister to bolster his flagging appeal. Despite the scale of the defeat in Crewe, many cabinet ministers still believe that Labour can win the next general election with the right leader. Miliband is said to have been persuaded by friends that even if the Conservatives win he can still perform a vital service to the Labour party by reducing their majority.

A spokeswoman for the foreign secretary said: “There is no leadership contest.”

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