Brown is sure to go, but who will take the reins?

Gordon Brown will fly south from Scotland tonight, not knowing whether he should call a removal firm to collect his belongings from Downing Street or resume the reins of government.

Until Thursday night, it appeared that the only certainty about this month's election was that it would end Mr Brown's political career. Getting him out was the Tories' main battle cry, echoed by pro-Tory newspapers, and even Nick Clegg had said that, though he might do a deal with Labour, it would not be with Gordon Brown.

But on Friday night, the Prime Minister had what his staff have described as an "amiable" phone conversation with the Liberal Democrat leader, who indicated that a deal with Mr Cameron is not sealed yet, making it still possible that the Lib Dems could turn to Labour. And a chastened Mr Clegg said nothing about demanding Mr Brown's head on a platter as part of the deal.

Instead, the only public call for Mr Brown's resignation yesterday came from a Labour MP, John Mann, who said that having him continue as PM would "rule out the credibility" of any Lab-Lib deal. Mr Mann said: "Thousands of voters told me that they were happy to vote for me, but it was not a vote for Gordon Brown. Many others stated that they would have voted for me if Gordon Brown was not the Prime Minister and Labour leader."

Yesterday morning, Mr Brown's political advisers, including Alastair Campbell, were back at their desks in Downing Street, and Mr Brown spoke on the telephone to leaders of other EU governments about the crisis in Greece, as if it were business as usual for the Labour government.

After lunch he went home to Scotland. Tomorrow, the Chancellor, Alistair Darling, will represent the UK at a conference of EU finance ministers, and other cabinet ministers are likely to go into their old offices to deal with immediate problems.

If David Cameron becomes prime minister during the week, it is assumed that Mr Brown will resign the leadership of the Labour Party as well as the premiership. It would trigger a leadership contest in which the 44-year-old Foreign Secretary, David Miliband, will run as the party moderniser in the Tony Blair tradition, backed by the Business Secretary, Lord Mandelson. The Schools Secretary, Ed Balls, 43, may offer what counts in current Labour politics as a more traditional programme.

Mr Balls, who is likely to be supported by the Labour Party's main financial backer, the Unite union, said in a recent interview with the New Statesman: "If I said I didn't want [to be Labour leader] you wouldn't believe me, but it's not what drives me."

Alan Johnson, the Home Secretary, who is 60 this month, may also stand, despite having seemingly ruled himself out in the past. "Alan has always ruled out running against Gordon because loyalty is a big thing for him," a close associate said. "But he was always talking in the present tense. He has never ruled himself out of a contest if Gordon stands down. This is a time when we need stability, and he could stand as a consensus candidate."

Another figure who could be significant in a leadership contest is the Dagenham MP Jon Cruddas, who came third in the deputy leadership election in 2007, with the backing of the big unions. A member of the Cruddas team said yesterday: "While David Cameron is in negotiations with Nick Clegg, people should sit back and see how those negotiations play out."

If Mr Brown leaves Downing Street, he could decide to quit the leadership of the Labour Party immediately, leaving his deputy, Harriet Harman, temporarily in charge. Ms Harman is thought to be willing to take over temporarily, but is not interested in holding the post for the long term. The alternative, which is less likely, is that Mr Brown would stay until a successor is elected. Mr Miliband and his backers are expected to push to have the leadership ballot quickly, because of the possibility that David Cameron will call an early general election.

Under party rules, if Labour's national executive decided on an early contest, it would need to hold a special party conference, which would add to the strain on the party's depleted bank account. Otherwise, the result would have to wait for the party's annual conference in late September.

"If we need to hold a one-day conference, I'm fairly sure the money could be found," a member of the executive said yesterday. "But the earliest it could be done would be July, because of the time it takes to organise a postal ballot."

Labour's future: Deal or no deal? How things may play out

Scenario 1

David Cameron becomes prime minister. Gordon Brown moves out of Downing Street and gives notice he will quit as leader of the Labour Party on 26 September, the opening day of Labour's annual conference in Manchester. Meanwhile, he has to face Cameron every Wednesday at Prime Minister's Questions, during which he is ritually humiliated. Labour's poll ratings slip as it spends four months engrossed in a leadership contest, with the ever-present risk that the Tories might call a snap election. Once the new leader is in place, Labour's ratings improve.

Scenario 2

Cameron becomes prime minister. Brown resigns as Labour leader with immediate effect. Harriet Harman takes over as temporary leader and, finding that she rather enjoys being in charge, persuades the National Executive that it can save a bit of money by delaying the leadership election until September. This gives her time to weigh up whether she should join the contest. Her weekly jousts with Cameron at Prime Minister's Questions are hit and miss. Labour's ratings fall until the leadership question is settled.

Scenario 3

Cameron becomes prime minister. Brown resigns, Harman takes over, but the National Executive insists it cannot risk a leadership contest overlapping an election should Cameron go back to the country in the autumn. It decides on a one-day conference in July when the results of a postal ballot can be announced, and the new leader and deputy anointed. Two months of intensive campaigning follow. In July, Labour has a new leader, who spends the August break planning strategy for the autumn.

Scenario 4

Brown and Nick Clegg come to an agreement. Brown gains a few extra months as prime minister, but announces his intention to resign the party leadership on 26 September, knowing his government is vulnerable to a vote of no confidence and that Labour must be in a position to fight an election under a new leader. Once elected, his successor enjoys a temporary boost in the opinion polls, and seizes the moment to call an election, remembering how Brown was punished in 2007 for not going to the polls.

Scenario 5

Brown realises a deal between Labour and the Liberal Democrats is possible, but not while he is prime minister. He calls a cabinet meeting and tells his colleagues he is resigning. The Cabinet has to choose a successor on the spot from one of their number. It turns to Alan Johnson as a caretaker leader, rather than making a contentious choice from one of the younger generation, such as David Miliband or Ed Balls. Johnson seals the deal with the Lib Dems.