Labour is braced for a backlash at the polls amid continued hostility from some of the lowest earners over the abolition of the 10p tax rate.
Moves to help the five million people hit by the change appear to have failed to reassure many on the doorsteps ahead of Thursday's local elections.
The issue comes before the House of Commons for another vote tonight.
Opposition politicians claimed most of those affected by the change would still be worse off even after last week's concessions.
Graham Stringer, the Labour MP for Manchester Blackley, said he still needed reassurances from Chancellor Alistair Darling about whether individuals would "get their money back or not".
"The announcements have been ambiguous to say the least as to whether people will be compensated, or when they will be compensated," he said.
The damaging row over the removal of the 10p lower tax band came to a head last week with Labour MPs threatening to defeat the Government in what would have been a damaging blow to Gordon Brown's authority.
But a rebel Labour amendment demanding compensation for those affected was withdrawn by its sponsor, Frank Field, on Wednesday after the Government came forward with concessions.
However, Liberal Democrat Treasury spokesman Vince Cable insisted that most of the five million would still lose out.
"Labour MPs have a stark decision to make - either they vote with the Government and make millions of low income workers worse off or they vote with their conscience," he said.
