Alistair Darling has risked the wrath of Labour's 10p tax rebels by ruling out quick concessions for low earners.

The Chancellor insisted that "unravelling" the financial package announced last year would be "irresponsible".

However, he did attempt to placate mutinous MPs by holding out the prospect of measures to help poorer Britons in the next Budget.

The Finance Bill that scrapped the 10p band is still before Parliament, and 70 backbenchers and six ministerial aides have expressed concern over the changes, saying the poorest will be hit.

Treasury sources have indicated that any scheme to redress the situation could cost up to £8 billion.

Meanwhile, the Labour infighting appears to be alienating voters, with the latest poll finding support for Labour had slumped by three points in less than a fortnight, with the party now trailing the Tories by 30 per cent to 40 per cent.

Mr Darling said on Sunday that the Government had an excellent record of helping low earners, having introduced tax credits and the minimum wage.

But he added: "In future Budgets, maybe in future Pre-Budget Reports, I also want to do more because I attach considerable importance to making sure that we help people on lower incomes," he said.

"What I cannot do is to rewind the Budget. It simply isn't possible as you go into a financial year to unravel the whole thing and attempt to rewrite it."

But former welfare minister Frank Field, who has been spearheading the rebellion, said the Chancellor's words were inadequate and promised he would force a Parliamentary vote.

Thurrock MP Andrew Mackinlay said the issue was "snowballing" and would seriously damage the party's results at next month's local elections.

The Tories - who have pledged opposition along with the Lib Dems - demanded that the Chancellor "get off his backside" and reverse the tax reforms.

"This was a pre-meditated attack on 5.3 million of the lowest paid people in our country, hammering them with an extra tax bill at a time when if they go shopping the grocery bill has gone up, if they fill up the car petrol and diesel has gone up," David Cameron said.

Foreign Secretary David Miliband has issued a plea for unity after weeks of vicious sniping at Gordon Brown's leadership.

Mr Miliband - often touted as a replacement for the PM - warned that electoral defeat was certain if the party "argued among ourselves, failing to defend each other and our leader".

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