By Nick Assinder Political correspondent, BBC News website

Tony Blair has just taken a major gamble with his already limited and shaky future as prime minister.

Gordon Brown has offered some support to Tony Blair

By setting a 12-month timetable for his resignation from Downing Street and saying he will name the day later, he dearly hopes to bring an end to what has without doubt been the greatest threat to his leadership since 1997.

He hopes those lined up against him will now pull back from the brink in the face of public scorn and the prospect of damaging Labour's hopes of future election victory - something he highlighted.

And he probably hopes that, what he has admitted will be his last Labour conference as leader will become a "thank you" rally for his past record rather than the scene of plotting and blood-letting it was threatening to become.

But in making his announcement he has been forced to do exactly what he has been battling to avoid since he revealed in 2004 that he would not fight a fourth election.

He has risked turning himself into a lame duck premier who has lost authority and control over his government and party.

And what may be worse for him is that, even with Gordon Brown's endorsement and call for unity, there is no guarantee his statement will end the turmoil that has engulfed him over the past few days.

Nods and winks

For example, Mr Brown made it plain he had "questions" over what will happen over the coming period, suggesting he was not entirely satisfied with the current state of play.

And by defiantly refusing to give a precise date for his resignation, Mr Blair risks further inflaming just those Labour critics who were spurred into action against him when the same reassurance came from David Miliband only days ago - and they were not all Brownites by any means.

These MPs simply do not believe Mr Blair can remain in power for another year and, without hearing an exact departure date from his own lips, they do not trust any nods and winks that he may go a bit sooner.

They want a date set in stone - and they want it to be long before May of next year when Labour faces hugely difficult local elections in England, Scotland and Wales.

Blair has taken a significant gamble

So, even if there is a private understanding between Mr Brown and Mr Blair over a specific date - something many doubt - it is very likely the issue will continue to dominate the period between now and Mr Blair's resignation.

While the prime minister's statement may bring some back from the brink, other MPs will continue claiming it is simply untenable for him to attempt to run the country, or introduce any radical, long-term policies in his last few months when the whole world knows his premiership is over.

Every time he attempts to introduce policies the question will be "what does Gordon think?" and there is the danger that he will be unable to carry any weight with domestic or foreign political leaders who will know they will soon be dealing with someone else.

Queen's speech

Yet, if Mr Blair finds it difficult to propose a fresh agenda or exert any authority, then he may as well not be in power at all.

Meanwhile, the opposition parties will use every opportunity to describe him as a dead man walking and seek to drive wedges between him, the chancellor and the Labour party.

Eyes will turn to the Queen's speech in November which is supposed to set out the legislative programme for the next parliamentary session.

Will it be Blairite or Brownite in flavour or will it simply be devoid of anything radical or significant, pending the handover of power?

And then, of course, there is the curse of events which will inevitably erupt from time to time and demand the sort of authoritative and decisive leadership that Mr Blair may now struggle to deliver.

It is perfectly possible however that, having witnessed the turmoil of the past few days, Labour will indeed now pull back and allow Mr Blair to complete the last few months of his premiership in peace.

Nick.Assinder-INTERNET@bbc.co.uk