The Minister of State at the Scotland Office has resigned, Downing St has confirmed.
David Cairns is the fourth MP to be sacked or decide to leave the government over the issue of the Labour party leadership.
He used to work for Siobhain McDonagh, who was sacked last week after calling for an election.
Mr Cairns is a former Roman Catholic priest and was therefore barred from election to the Commons under two 19th century laws.
Ms McDonagh introduced the House of Commons Disqualification (Amendment) Bill in Parliament 1999 but it did not pass.
The government brought forward its own legislation, and the House of Commons (Removal of Clergy Disqualification) Bill was passed in May 2001.
Mr Cairns was elected as MP for Greenock and Inverclyde at a month later. He has been a government minister since 2005.
He voted in favour of gay adoption, civil partnerships and the abolition of Section 28.
Gordon Brown’s decision last week to sack Ms Mc Donagh as a government whip opened up the issue of whether Labour should have a leadership election.
More than ten MPs have written to the party’s general secretary to try to trigger the process, but he has refused to send out nomination papers to all of the party’s MPs.
Joan Ryan, a party vice-chair and the Prime Minister’s envoy to Cyprus, was sacked on Sunday for speaking out and the PM’s forestry envoy, Barry Gardiner, has resigned.
The rebels hope to break open the issue ahead of next week’s party conference in Manchester, by putting the nomination papers into the hands of MPs.
If 20% of them nominate an alternative candidate, and he or she accepts the nomination, then the issue will go to conference.
70 of the 350 Labour MPs would need to nominate a candidate.
The chair of the party’s National Executive Comittee, Dianne Hayter said today:
“Only Labour MPs can trigger the process and the NEC is confident that most MPs know their responsibilities under the rules.”
However, one leading rebel, former Home Office minister Fiona MacTaggart, was unreprentant.
“Previously I’m afraid that they were behaving somewhat like ostriches with their heads in the sand, hoping things would get better and turn up,” she told the BBC.
“That is not now possible.”