Alistair Darling risked the wrath of Labour's 10p tax rebels by bluntly ruling out quick concessions for hard-hit low earners.
Despite festering anger on the Government benches, 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 comments came as Foreign Secretary David Miliband 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".
Business Secretary John Hutton, known as an arch-Blairite, also admitted that many Labour MPs were "aiming at their own goal". "They have got to stop doing that," he told Sky News' Sunday Live programme.
But there was little sign that the entreaties were having any effect on those enraged by the decision to abolish the 10p income tax band.
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.
"The idea that somehow we'll do something undefined in the future to protect the poorest people in work, just is not on for most Labour backbenchers," he told BBC News 24.
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Thurrock MP Andrew Mackinlay said the issue was "snowballing" and would seriously damage the party's results at next month's local elections.
