As recently as Thursday of last week, close allies of the Health Secretary insisted he would not run for the job of mayor. They suggest that his mind was changed by three events.

Frank Dobson's job is to stop first Ken Livingstone and then Lord Archer

Firstly, the encouragement he received from party members at Conference to stand. Secondly a disappointing response to the candidacy of the Greenwich MP Nick Raynsford, when he announced his candidature.

And, most surprisingly, the decision of the Conservatives to select Lord Jeffrey Archer to fight the mayoral election.

Whatever his allies may say, many in the Labour Party believe Mr Dobson has been under intense pressure from those shadowy so-called "senior figures" in the party we read so much about in the newspapers.

Certainly, the alternative scenario being advanced by some is that Mr Dobson already knew that he would be moved from health in the forthcoming cabinet reshuffle, and possibly sacked altogether.

The announcement is causing particular anger amongst supporters of Mr Raynsford. They say he spoke to Mr Dobson only the day before declaring, and had made it clear that he wouldn't run against the health secretary. But for now, his camp insist that the former minister for London will not stand aside.

Nick Raynsford: Received assurances Dobson would not stand before declaring himself

In opposition, the two men worked closely together in formulating the party's policy for London. It's frequently been reported that Mr Dobson did not support the idea of a directly-elected mayor, preferring one elected by members of the new London Assembly. Mr Raynsford, by contrast, supported it, and has been instrumental in putting it into law.

That will be one criticism the health secretary will face. The other will be the perception that he's been bounced into standing. The prime minister is shrewd enough to make sure his fingerprints are not on this decision.

(He learnt that lesson from the debacle over apparently forcing Alun Michael to stand against Rhodri Morgan for the Labour leadership in the Welsh Assembly).

But he will be delighted that a seasoned campaigner like Mr Dobson is now in the race. There were real fears that none of the Labour candidates would be able to defeat the left-wing Labour MP Ken Livingstone to the party nomination.

What of those other candidates? So far Mr Raynsford is staying put. But supporters of Glenda Jackson say she will consider her position. She's an old friend of Frank Dobson, and MP for Hampstead and Highgate, which is next door to Mr Dobson's Holborn and St Pancras constituency.

Trevor Phillips: Could step down in Assembly list deal

The broadcaster Trevor Phillips is also staying in the race for the Labour nomination so far. But some believe he could be offered a deal, whereby his name is at the top of Labour's list for the London Assembly if he drops out of the mayoral campaign.

This would leave a head-to-head between Mr Dobson and Mr Livingstone. Certainly, it now seems much less likely that Labour will block the Livingstone candidacy. This will be a relief to many party members in London. Even Blairite loyalists have warned the leadership of a backlash if there was any sense of a stitch up.

But this is still a risky strategy. Both men are long-standing London figures, with local government experience. They were councillors together in Camden in the 1970s. There's no guarantee that Mr Dobson could win in a straight fight, still less so if there's any suggestion that he's been bounced into standing by the party leadership.

Next week, Labour will formally announce the procedure for choosing its candidate for Mayor of London. After that, the gloves will be off.

That still leaves one more intriguing possibility. If Mr Dobson wins, but come next year opinion polls still suggest Mr Livingstone is ahead, will he consider jumping ship and standing as an independent?

He's denied it consistently, but then he has to. An official Labour versus an independent Labour battle could then have the effect which terrifies many loyalist Labour members more than the idea of a Livingstone victory - Lord Archer as Mayor of London.