Labour recriminations over the Crewe & Nantwich by-election have started three days before polling, with supporters of Gordon Brown distancing him from the “class war” campaign that has dominated it.

As a poll last night gave the Conservatives a 13-point lead in a seat that the late Gwyneth Dunwoody won by 7,000 votes in 2005, senior ministers and officials voiced regret that attempts to depict the Tory candidate as a “toff” had backfired. The tactic prevented Labour’s candidate, Mrs Dunwoody’s daughter Tamsin, from getting her case across, they admitted.

The ComRes poll for The Independent suggests that Labour is heading for a heavy defeat. Taken between Thursday and Sunday, it put the Conservatives on 48 per cent, Labour on 35 per cent and the Liberal Democrats on 12. Worse for Mr Brown, it found that only 13 per cent of people in Crewe regard him as an asset to his party, while 60 per cent see him as a liability.

An ICM poll in The Guardian last night put Labour on 27 per cent nationally and the Conservatives on 41 per cent, the worst showing for Labour since May 1987, just before Margaret Thatcher won her third election by a landslide.

The Times has been told that Mr Brown will not carry out a Cabinet reshuffle if Labour loses badly, but will treat it as an outcome that could be expected in the middle of a third term and with national polls heavily against the party. To underline the “business as usual” message, it was disclosed that Mr Brown plans to spend the Whitsun Bank Holiday weekend in Scotland with his family.

Among some Labour MPs, the knives are out for Steve McCabe, the whip and MP for Birmingham Hall Green, who was chosen by party headquarters to run the campaign. He had been associated with the disastrous Dunfermline & West Fife by-election in 2006, in which the Liberal Democrats took the seat from Labour in Mr Brown’s backyard.

At the same time, Mr Brown’s allies are braced for “ultra-Blairites” to make a fresh attack on his leadership on Friday if the result turns out badly.

In London, strategists say that the campaign became “bogged down” with the attacks on Edward Timpson, a barrister from the shoe business family, as a member of the upper classes. “But having made their point, they should have admitted it was a stunt and moved on. Instead, we have been diverted into this row over whether Labour is now for or against aspiration,” one said.

The acrimony between the parties deepened last night after it was claimed that campaign posters had been ripped down by rival activists. Tory sources claimed that two men had been stopped by police and found with “piles of our posters” in a vehicle.

Labour officials then countered with a claim that they had filed a police complaint after witnessing people in a white van ripping down their posters. Police would not confirm last night that they were investigating the claims.

Mr Brown and the Cabinet are desperate to avoid the Crewe result sparking a “nightmare” weekend similar to that which followed the local elections. A senior strategist said: “After the locals it was right to have a serious inquest and take action because of what the country was telling us. That is why we acted on the 10p tax. But there will be no repeat of the internal soul-searching. Crewe was always going to be tough in midterm and commentators who pretend that we should be expecting to win this seat at this time are being dishonest.”

Even so, it is clear that Mr Brown is ready for another difficult Thursday night and Friday. Senior ministers do not expect any move to be made against him and believe that he must be given time to recover from the 10p debacle. The Commons will be in recess when the Crewe result is announced in the early hours of Friday and Labour MPs will have a week to calm down. But ministers say that if Labour’s position has not improved by next year’s local elections, all bets would be off.

Labour’s backroom campaign team remained unrepentant last night about portraying Mr Timpson as a “Tory toff”. They insisted that it has played well on the doorstep in Crewe.

One strategist said: “It has been a real focus of the campaign whether Edward Timpson is fit to to be an MP for the people of Crewe & Nantwich.

“We have made no apologies for the stuff we have done regarding making it clear that Tamsin Dunwoody is one of us, somebody who understands the concerns of local residents and asking the question whether somebody with Mr Timpson’s background can do the same. He tries to paint himself as a local person but he is a world away.”

Earlier Ms Dunwoody was anxious to emphasise the class distinctions between herself and Mr Timpson. “The difference between us is that I do not have a £53 million family fortune supporting me. I do not have a £1½ million mansion. I am just a single, unemployed mother of five fighting hard for a job.”

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