MERSEY MP Stephen Hesford has vowed to quit the government unless Gordon Brown backs down over plans to sell off part of Royal Mail.
The Wirral West MP has threatened to sacrifice his unpaid post as a ministerial aide by voting against the Bill to hive off up to 30% of the business to a foreign postal operator.
Asked if the protest would be mean resigning, Mr Hesford said that would be for others to decide, but added: "If that was the consequence, that would be the consequence."
The threat - and similar protests by up to seven other parliamentary private secretaries (PPS) - came as Mr Brown was warned that pressing ahead with the legislation would be a "kamikaze move".
Nearly 150 Labour MPs, including a further eight from Merseyside and North Cheshire, oppose the part-privatisation, warning that the Royal Mail’s "universal service" is at risk.
The scale of the revolt would force the prime minister to rely on Conservative support to win a crunch Commons vote on the Postal Services Bill, expected in early June.
Some Labour MPs fear Tory leader David Cameron would back Mr Brown at second reading, only to withdraw support at a later stage of the Bill - forcing Labour into an unwanted early election.
Meanwhile, there were suggestions that the ferocity of the opposition is making companies think again about bidding for a stake in running Royal Mail.