Frank Field wants more details on the compensation on offer to tax losers
The leader of Labour's backbench revolt against scrapping the 10p tax rate has threatened to revive the protest.
Former minister Frank Field urged Gordon Brown to specify how those affected will be compensated.
Mr Field told the Mail on Sunday unless Prime Minister Gordon Brown made the package "crystal clear" several MPs were prepared to block the Budget.
Mr Brown said tax was complicated but he thought they could "do something" for the low paid and 60-64 year olds.
Mr Field blamed Labour's poor showing in this week's local council elections in England and Wales on the 10p row.
'Clear as mud'
Last month he withdrew an amendment to the Finance Bill - which had been supported by 46 Labour MPs - after being told the compensation scheme would be backdated to the start of the financial year.
But he said the proposal to offset losses from the axing of the rate through the winter fuel allowance, tax credits and minimum wage had been "clear as mud".
"I believe that the prime minister will be as good as his word, that the package will cover as many of the 10p losers as possible and that it will be fully backdated," he said.
Mr Field added: "I, along with a number of MPs, believe that if ministers fail to make crystal clear how the package is taking shape, we should retable our Commons amendment on the 10p tax rate, and so block the Budget."
The Birkenhead MP said the government had "rushed into" the Crewe and Nantwich by-election caused by the death of Gwyneth Dunwoody which could have been held in six months' time.
He said: "The government must learn the lessons of its ineptitude in handling the 10p tax controversy.
"And it needs to do so in double-quick time - for it has set itself another, immediate, electoral hurdle."
Speaking on BBC One's Andrew Marr Show later, the prime minister said he believed Chancellor Alistair Darling had done enough to satisfy Labour's tax rebels.
"Nobody's suggesting that the 10p rate will be brought back, not any of the opposition parties, not Frank Field.
"He knows, as I know, that it was a transitional measure until we introduce the lower rate income tax and the tax credits that are the basis of taking thousands, and indeed millions of families out of poverty."
He said he thought Mr Field had been "satisfied by the government's response.
He added: "Tax is a very complicated issue but we think we can do something for the 60-64 years olds and we think we can help the low paid."