Senior Labour figures have warned that Gordon Brown could face a delegation of “men in suits” pressing him to resign if the party loses the pivotal Crewe by-election this week.

With the campaign descending into accusations of dirty tricks, discontent with the prime minister’s leadership has begun to spread.

“There has been a shift in mood,” said one MP. “Mutterings about regime change used to be confined to the fringes of the party, but now there is a more widespread view that Gordon will have to go unless he can win back public support.”

Labour has been accused of deploying increasingly desperate tactics to hold on to Crewe and Nantwich in Cheshire. The party is defending a majority of 7,078 in Thursday’s vote.

Yesterday Labour issued a new leaflet claiming that Tamsin Dunwoody, its candidate, had helped to persuade Alistair Darling, the chancellor, to change his mind over the 10p tax debacle. Contradicting ministers’ repeated claims that last week’s mini-budget had nothing to do with the by-election, the flyer states: “Tamsin delivers for Crewe and Nantwich. You told Tamsin your concerns. She put them directly to the chancellor. She stood up for you.”

There have also been Labour attempts to smear Edward Timpson, the Conservative candidate, as a “friend of the paedophile” because he has occasionally defended sex offenders in his job as a barrister. “I think you will find he is not the type of lawyer he claims to be,” one Labour MP said.

In further evidence of negative campaigning, Labour activists have been accused of telephoning Crewe voters in the middle of the night posing as Conservative canvassers. A Tory campaign source said: “It would not surprise us if Labour was stooping to this level. Its entire campaign has been marked by mean-spirited stunts and dirty tricks.”

If Brown is forced to stand down before the next general election, James Purnell, 38, the work and pensions secretary, could emerge as frontrunner. His leadership campaign is likely to receive support from Stephen Carter, Brown’s most senior political adviser. “Carter tells anyone who will listen that James is the best thing since sliced bread,” a Labour source said.

No 10 is braced for further personal attacks on the prime minister if Thursday’s vote goes against Labour. A senior party figure said: “One way of unseating him would be through a war of attrition, with relentless carefully placed attacks laying the ground for a delegation of elder statesmen – the ‘men in suits’.”

Allies insist the prime minister’s resilience should not be underestimated. A leaked strategy document reveals the blueprint for the No 10 fightback.

The paper, by David Muir, Downing Street’s director of political strategy, finds that private focus groups have compared David Cameron, the Tory leader, with an estate agent from Foxtons, the London firm famed for its aggressive sales technique. The polling suggests the public sees Cameron as energetic but believes there is “something about him that is not quite right”.

“Asked what he would be in another life, it would be a salesman, car dealer, PR guy, estate agent and, worst of all, a Foxtons estate agent.”

The document also says the government has lost its “licence to be heard” and needs “quick wins”. In an attempt to boost morale among despondent Labour activists, the document says there is “no great warmth” towards Cameron and the Tories, claiming that the opposition is “strategically brittle”.

However, it counsels against class-based attacks – which Labour has used with its activists wearing top hat and tails in Crewe – saying the public is “very hostile” to this approach.

Last night an ICM poll of Crewe & Nantwich voters put the Conservatives on 45%, eight points ahead of Labour on 37%.

Observers in Crewe say a glamorous outsider candidate could win enough votes in the by-election to have a significant impact on the outcome. Gemma Garrett, otherwise known as Miss Great Britain and candidate for the Beauties for Britain party, is said to have become a focus for disaffected Labour voters.

It emerged that Garrett has been given tips on how to run her campaign by Shaun Woodward, the Tory defector who is now Northern Ireland secretary.

The pair met at the Ivy restaurant in London several days ago, after being introduced by friends.

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