Gordon Brown said during the General Election campaign: "If I couldn't make a difference any more, then I would go off and do something else."
If Mr Brown steps down in the face of a Tory-Lib Dem deal for power, the following figures would be among the favourites to succeed him in what would inevitably be dubbed the battle for the soul of the Labour Party:
* Harriet Harman - As deputy leader of the party, she would automatically take over temporarily if Mr Brown stepped aside before his successor was chosen.
The former solicitor general, Leader of the House and minister for women and equality, surprised many by her victory in the ballot to be Mr Brown's second-in-command, winning through by scooping up second preference votes in the party's electoral college system.
Few would bet against her doing so again, even if only as a holding leader while the party determined its ideological future. Ms Harman, 59, is married to Unite deputy general secretary Jack Dromey, who also stood as Labour candidate in Birmingham Erdington.
* Alan Johnson - As a former joint General Secretary of the Communications Workers Union and holder of heavyweight Cabinet posts from Health Secretary to Home Secretary, Mr Johnson, 59, was often portrayed as the man most likely to succeed Mr Brown when the premier was being criticised for seeming aloof and lacking the common touch.
He has made great play of the need for electoral reform alongside the shake-up of the Westminster expenses system, saying last year: "We need to overhaul the engine, not just clean the upholstery."
The ex-postman would also hope to garner valuable union support.
* David Miliband - Nicknamed "Brains" by Tony Blair's former spin chief Alastair Campbell, he would be seen categorically as a Blairite contender for the leadership, ready to wrest the reins of the party back from the Brownite wing.
The Foreign Secretary, 44, is said by some commentators to have ducked taking on Mr Brown in the past with leadership challenges which could have ousted him before the General Election.
He was also famously photographed looking more awkward than usual clutching a banana outside Labour's party conference.
As the former head of the Downing Street Policy Unit under Mr Blair, he has experience of the sharp end of life in No 10, but tribal loyalties could count against him.
* Ed Miliband - Brother of David, the 40-year-old Climate Change Secretary was previously a key advisor to Mr Brown. He is seen as more easy-going and fluent in public than his older sibling.
He was first elected as an MP in 2005, the same Westminster generation as Liberal Democrat leader Nick Clegg, and has in the past been tipped for high office just as often as David - whose appointment at the Foreign Office was widely seen as a peace offering to the Blair camp.
As probably the youngest of any contenders he could hold out the hope of a real fresh start for his party - but would he be happy slugging it out at the hustings with his brother?
* Ed Balls - Once Gordon Brown's closest economic advisor at the Treasury, the Children's Secretary would be seen as the premier's chosen successor and would undoubtedly square up to David Miliband if the contest reverted to tribal New Labour loyalties.
Mr Balls, 43, was also one of the 2005 intake of MPs and would be expected to gather support from Labour's powerful union backers, especially the Unite union.
His combative, sometimes abrasive, style would ensure a fiercely-fought contest. But he may suffer from being seen as the heir of a man who had chosen to "go off and do something else".
* Jack Straw - Now the closest that Labour has to an elder statesman in the Commons, the Justice Secretary, former home and foreign secretary, would definitely be viewed as a holding leader.
His appeal could be to offer a period of calm, while the party debated its future policy direction in opposition or government.
Mr Straw, 63, would probably have to be elected almost by consent rather than through fierce debate - or could come through the field as opponents cancelled each other out.
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