Despite his protestations, the PM's destiny has never been in more doubt

Until last night, Tony Blair had always sidestepped inquiries over when he might vacate Downing Street. He would explain that anything other than the most non-committal of replies would lead either to headlines dismissing him as a lame duck or recalling Margaret Thatcher's boast she intended to go "on and on".

Now there is a clear end in sight to the Blair era that has transformed the Labour Party, bringing with it unprecedented electoral success, but also taking it in directions - such as the dogged support of a right-wing Republican American President in a foreign war - that have strained the loyalty of many of its traditional supporters.

The Independent's disclosure that the Blairs are buying a private home in central London shows that the couple are actively preparing for life after government.

The Prime Minister's admission that he is being undergoing a heart operation today is also bound to add fresh speculation over his literal fitness - and appetite - for the job.

"It was a bit of fluttering," he told the BBC last night, "it does not stop you working, I've been working for the past couple of months since it happened ... It's just something you should get fixed. I have been feeling fine but it's as well to get it done."

Referring to the procedure he is due to undergo today, he added: "Its with a local anaesthetic. Anyway, it fixes it , which is the main thing."

With the Labour Party in fractitious and introverted mood, Mr Blair's dramatic announcement - ten months after he brushed off a previous heart flutter a "a moment's illness ... translated into a major crisis" - will reopen the speculation over when he might step down.

There had been hints of his political mortality over the past six months. Rumours were widespread that he would resign if the Government lost January's cliff-hanger vote on university top-up fees.

Five months later, reports surfaced that four Cabinet Ministers had urged him to stay on in the job after he suffered a "wobble" of confidence.

John Prescott, the deputy Prime Minister, inadvertently reopened the speculation in the summer when he spoke of the "[tectonic] plates moving" at the top of the party.

Assuming Labour wins the next election - something less easy to take for granted as the Government struggles through one debilitating crisis after another - Mr Blair will never be able to escape the "when?" question.

Aides tried to scotch that problem last night by insisting he would serve most of a third-term. However, the reality is that every interviewer he faces will turn at some stage to the potential date of his retirement and every speech he delivers will be pored over for clues of his intentions.

His announcement also inevitably reopens the question of who will inherit the crown.

Will this finally be the moment, many years after the infamous Granita deal between Mr Blair and Gordon Brown, that the Prime Minister honours a promise to hand over the succession?

The Chancellor will inevitably be the favourite for the job, having spent years assiduously cultivating unions, constituency parties and MPs who will choose the party's next leader - and probably the country's next Prime Minister.

There is little doubt Mr Brown's notorious ambition for the job remains undimmed and, as always, his address to the Labour conference this week was a reminder of the strength of his performance at the Treasury. But there can be no assumption that he would take over unchallenged. Were Mr Blair to carry on for the bulk of the next Parliament, the Chancellor could be 57 years old and the feeling could spread that the time had come to pass the baton to a young man or woman.

Alan Milburn's return to the Cabinet this month marked him out as the most likely "Blairite" challenger for the job - and probably the preferred candidate of the retiring Prime Minister. Alternatively a younger alternative, possibly Ed Milliband, may push forward to the front of the pack

Mr Brown could also face a challenge from cabinet rivals such as David Blunkett, Charles Clarke and John Reid, all of whom have been touted as alternative leaders.

In the shorter term, there will be questions over how long it will take Mr Blair to recuperate from his operation. Will he be able to undertake his planned trip next week to Africa? Will he be available if the Northern Ireland peace process needs his intervention?

The uncertainty provides an unexpected boost to Michael Howard, whose Conservative party conference opens next week. The Tories are bound to argue that a vote for Blair in next year's general election would ultimately deliver a Brown regime.