Labour rebels last night claimed they had the “bit between their teeth” after humbling Gordon Brown into the biggest policy U-turn of his premiership.

Ministers warned that their backbench MPs smelt blood after forcing the Prime Minister to rewrite a defining measure of his last Budget as Chancellor to buy off the 10p tax revolt.

Rebels - sensing that the climb-down had sapped Mr Brown’s authority - threatened to exploit his weakened position, saying that he faced defeat over his plans to detain terrorist suspects for 42 days unless he listened to their concerns.

Mr Brown last night denied that he had been pushed around after an extraordinary, corrosive and crisis-ridden few days at Westminster. For the first time his future was questioned by some MPs privately.

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Will Labour MPs now scent that when it comes to rebellions Mr Brown can be a bit of a pushover?

Philip Webster

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Despite continually playing down the impact of his 2007 decision to scrap the 10p rate of tax, he blinked first in a trial of strength with backbenchers.

After suggesting initially that there could be no concessions, Mr Brown and Alastair Darling first promised unspecified help in the autumn, then said that it would come in this financial year and finally said yesterday that the package of help, which remains vague but will cost hundreds of millions of pounds, would be backdated to April 1. MPs who signed up to an amendment tabled by Frank Field requiring the Treasury to come forward with compensatory measures, claimed “game, set and match”.

Mr Field, who had had a private meeting with Mr Brown the night before to lay down the rebels’ “bottom line” demands, withdrew his amendment, stating that he had got what he wanted. He even claimed that “phase two of Gordon Brown’s premiership” - one in which he was big enough to admit he had made mistakes – had arrived.

Mr Brown, facing the Commons only minutes after the concessions were made, was cheered by Labour MPs relieved that they could go back to fighting the local elections able to show that the Government had listened to constituents.

But many MPs admitted that, having been forced to act by the fear of a Commons defeat, the Prime Minister had been wounded.

No one will ever know whether the rebels rather than Mr Brown would have blinked, understanding that to defeat Mr Brown on the Finance Bill would have triggered a genuine crisis of confidence in him.

David Cameron accused Mr Brown of a “humiliating” U-turn that meant a “massive loss of authority”. He said that Mr Brown had caved in.

But in a series of television interviews, Mr Brown insisted: “I don’t think I’ve been pushed about at all. What I’ve done is listen and made the right long-term decision.”

He emphasised that the “fundamental” change of scrapping the lowest income tax band was still going ahead.

The U-turn was announced in a letter from Mr Darling, who said that losses suffered by low-paid workers without children and pensioners aged 60-64 would be offset through the winter fuel allowance system, tax credits and the minimum wage. Crucially for the rebels, he said that the help would be backdated.

In the Commons Mr Brown said that the 10p rate was not the best way to boost the incomes of the poor, who had benefited from improvements in tax credits and the minimum wage under Labour. “The central issue is that we have taken more people out of poverty than any previous Government,” he said.

The cost of the package is unknown but is likely to run into hundreds of millions rather than the estimated £7 billion that it would cost to restore the 10p rate. Eligible pensioners are expected to receive a one-off payment on top of their winter fuel allowance, while the Low Pay Commission will advise the Chancellor on what changes could be made to the minimum wage to help younger workers.

Last night MPs weighed the implications for the 42-day vote. Ian Gibson, MP for Norwich North, said: “We are buoyed up. In the end they listened and we hope they will listen again.”

Another senior MP said: “This has shown a number of us that the system in this place actually works. We’ve got the bit between our teeth. It’s democracy at work.”

Of the 42-day vote, Mr Brown said: “It’s right on security issues, because you never compromise security issues, that we should go ahead with 42 days.

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