Five ministerial aides have joined the Labour rebellion against Gordon Brown's decision to abolish the 10p starting rate of income tax.
The calls for a rethink by normally loyal supporters will add to the pressure on the Prime Minister to make a partial retreat. Although ministers say they will not reverse the decision, they may be forced to pledge compensation for some of the 5.3 million people on low incomes who will lose out.
Only a personal plea by Mr Brown, who had to interrupt talks at the White House, averted the resignation of Angela Smith, a Treasury aide. It emerged yesterday that at least four other parliamentary private secretaries have expressed concern.
David Anderson, aide to the Higher Education minister Bill Rammell, has signed a Commons motion criticising the move.
Celia Barlow, aide to the Science minister Ian Pearson, has written to Mr Brown about her concern. A fifth aide, Nia Griffiths, who works for the Environment minister Phil Woolas, tackled Mr Brown on the issue at a meeting of Labour MPs this month.
MPs will discuss the issue on Monday and a vote is expected a week later.
Frank Field, the former minister who will table an amendment to the Finance Bill demanding help for those who will lose out, said: "We have never been asked [before] to vote for a package that will make five million of the poorest people worse off. There is no doubt among Labour members that they did not go into Parliament to do this."
Ed Balls, the Schools Secretary, said Labour must "hold our nerve" and remain united. Writing in Tribune magazine, he admitted Labour had had "a tough few weeks" and it would "get tougher" with local elections on 1 May. But he said "doom merchants" needed a reality check.