Although cabinet ministers said there would be no immediate attempt to oust the Prime Minister, some predicted he would face a concerted move to force him to stand down in September – possibly before the Labour conference, which starts on 20 September.
Ministers acknowledged that the disastrous and unexpected defeat at the hands of the Scottish National Party, by 365 votes, could prove to be a "tipping point" for Mr Brown. "The mood is changing," said one source in the Cabinet. "It is starting to cement."
Indeed, the shift is so stark that cabinet ministers are preparing for a possible leadership contest. And The Independent has learnt that James Purnell, the Work and Pensions Secretary, and a rising star in the Cabinet, would not stand against David Miliband, the Foreign Secretary, should he decide to run if Mr Brown were to depart. The move has echoes of the pact between Mr Brown and Tony Blair after John Smith, who was Labour leader, died in 1994.
To add to Mr Brown's woes, the latest monthly survey by ComRes for The Independent gives the Conservatives a 22-point lead over Labour, the biggest they have ever enjoyed in a ComRes poll. It puts the Tories on 46 per cent (unchanged on last month), Labour on 24 per cent (down one percentage point), the Liberal Democrats on 18 per cent (unchanged) and other parties on 12 per cent (up one point). The figures would give David Cameron an overall majority of 236 if repeated at a general election.
Backbenchers urged the Justice Secretary, Jack Straw, the Cabinet's elder statesman, and Geoff Hoon, the Chief Whip, to tell Mr Brown: "The game is up. We can't win an election under you."
While both men want to avoid frenzied leadership speculation, they are likely to test the party's mood next month after they return from their own holidays. Mr Straw faces an agonising dilemma because he is a possible "compromise candidate" to succeed Mr Brown.
Some backbenchers threatened to organise a round-robin letter calling on the Prime Minister to quit if the Cabinet failed to move against him. They say a cabinet move would be less bloody and are urging its senior members to "show leadership".
Some senior Labour figures and ministers are not yet convinced there is a leader in waiting who could revive Labour's fortunes. But others believe a change of leader could prevent a Labour massacre at the next general election.
Allies of Mr Brown insisted he would not be forced out. He plans to fight back with a package of economic measures to help people struggling with their housing and energy costs.
"My task is to get on with the job of getting us through these difficult economic times," he said yesterday. Later he told a meeting of Labour's national policy forum at Warwick University: "There is nothing bad about Britain that cannot be corrected by what is good about Britain."
Tony Lloyd, the chairman of the Parliamentary Labour Party, insisted: "The answer to this isn't looking for Gordon Brown's scalp."
Mr Cameron called on Mr Brown to end the uncertainty by calling a general election. "I think we need change in this country, and that's how change should come about," he said.
Alex Salmond, the SNP leader and Scotland's First Minister, said the voters had sent a clear message to Mr Brown. "We have now demonstrated that there are no safe seats for the Labour Party anywhere in Scotland," he said.
According to ComRes, the Conservatives are successfully mobilising their core vote. Some 96 per cent of people who think of themselves as natural Tories plan to vote for the party, compared with 77 per cent of natural Labour supporters and 82 per cent of Liberal Democrat identifiers.
Some 67 per cent of people who intend to vote Tory say they are "absolutely certain" to vote, compared with 61 per cent of Labour supporters and 55 per cent of Liberal Democrat backers.
ComRes telephoned 1,021 adults on July 23-24. Data were weighted by past vote recall. ComRes is a member of the British Polling Council and abides by its rules. Full tables at www.comres.co.uk
Who's in the running?
David Miliband odds: 5/2
The Foreign Secretary is seen by many as Labour's heir apparent. The former No 10 policy chief and rising star of the Cabinet came under pressure to stand as a Blairite leadership candidate against Gordon Brown.
Jack Straw 6/1
As Justice Secretary he is seen as an effective deputy prime minister and is coming under pressure to lead a cabinet delegation telling Mr Brown to go. Would be keen to take over if only as a short-term caretaker.
John Denham 7/1
Potentially attractive to middle-class voters. He has been praised by some Labour MPs as a possible unity candidate, and could attract support for his stance against the Iraq war. He has few enemies in the party.
Jon Cruddas 10/1
The MP for Dagenham won admirers for his deputy leadership campaign, in which he came third – ahead of three cabinet ministers. He turned down a job when Mr Brown arrived in No 10. That could count both for and against him.
Alan Johnson 5/1
Praised for his down-to-earth style and sure-footed handling of the NHS as Health Secretary. He has been tipped for a deputy prime ministerial role, but appears reluctant to stand for the top job.
James Purnell 7/2
Seen as the next Labour leader-but-one. The 38-year-old public school product could appeal to Home Counties voters. Detractors say he can be a little too self-satisfied.
Ed Balls 10/1
Mr Brown's closest confidant in the Cabinet and the PM's choice as his successor. The close link with Mr Brown, and his slightly awkward manner, could be his biggest problems in a leadership bid.
Harriet Harman 16/1
She has ruffled feathers in Downing Street by suggesting she will be in charge while Mr Brown is on holiday, but she has performed strongly when standing in for him in the Commons.
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