Gordon Brown was facing a critical test of his faltering authority last night as Cabinet warfare erupted over plans to sell off part of the Royal Mail.
One of the Prime Minister's key allies, Chief Whip Nick Brown, was accused of encouraging Labour rebels to oppose the part-privatisation, a pet project of Business Secretary Lord Mandelson.
The row came as Harriet Harman, one of those said to have voiced doubts about the sell-off, sought to shore up the embattled Prime Minister.
Unlike Health Secretary Alan Johnson, she ruled out entering a leadership contest in the future.
In a frantic round of TV and radio interviews, the deputy Labour leader said she would not seek the top job even if Mr Brown were to step aside.
'I am saying there are no circumstances,' Miss Harman said. 'I do not want to be Prime Minister, I do not want to be leader of the party.'
Yesterday, one Cabinet minister told the Daily Mail that Nick Brown appeared to be actively trying to sabotage the Royal Mail proposal.
The minister said: 'People are being told that they can sign the Commons motion opposing it because the vote is not going to happen. He has also been inviting rebel MPs to come up with alternative ideas.
This sort of behaviour is quite extraordinary for the Chief Whip. If he carries on, he'll have to go.'
Modernisers in the Cabinet are now gunning for the Chief Whip, who is supposed to be in charge of party discipline.
They insist the Prime Minister should force the Royal Mail measure through the Commons next month even if it means relying on the support of the Tories.
But Left-wing ministers are urging him to shelve the sell-off, arguing he cannot
afford a row with 150 Labour MPs who have signed a Commons motion opposing the move.
They also fear pressing ahead leaves Mr Brown's fate in the hands of Tory leader David Cameron.
If Mr Cameron pulled the plug on his party's backing for the Royal Mail sell-off at the last minute, the Prime Minister would face a humiliating defeat that could force him to quit Downing Street.
Mr Brown looks likely to be forced to offer a compromise, which could see Royal Mail workers become part-owners of the new business on the model of the John Lewis department store.
A full-scale U-turn could risk a dangerous backlash against the Prime Minister from Cabinet modernisers - hot on the heels of an expected drubbing for Labour in next month's European and local elections.
Last night party sources sought to dampen already dismal expectations for the June 4 polls.
'This set of elections are the most challenging faced by the Labour Party for electoral structural reasons, never mind the political or economic context, since 1993 or indeed before,' one said.
'The good thing is that they have proved to be inadequate pointers to any subsequent General Election results.'
Miss Harman's intervention yesterday came after a desperate week that saw Mr Brown suffer his first Commons defeat on immigration rights for Gurkha veterans.
He was also forced into a series of climbdowns over reforms to MPs' expenses.
Then Communities Secretary Hazel Blears launched an extraordinary attack over Mr Brown's 'lamentable' failure to communicate, mocking his gauche appearance on the YouTube website.
Miss Harman's categoric statement on her leadership ambitions appeared calculated to contrast with Mr Johnson's stance.
He astonished colleagues at the weekend by giving a broad hint that he would accept the job if Mr Brown's premiership collapses. Allies of the Prime Minister fear that while wellintentioned, Miss Harman's move could prove counterproductive.
By killing speculation about her intentions, she may strengthen Mr Johnson's claim as a successor who could be painlessly anointed.
A key argument against removing Mr Brown has been the prospect of weeks of civil war as contenders battle it out to replace him.
But of three ministers touted as potential 'caretaker' leaders - Miss Harman, Mr Johnson and Justice Secretary Jack Straw - only Mr Johnson has now failed to rule himself out.
A Downing Street source said: 'Nothing has changed on Royal Mail. The legislation is going through the Lords and will come to the Commons later in June.'