Focus: If not now, when? Only one man can answer, in his own good time

Cherie might welcome it, Gordon might rejoice in it and his growing band of critics might take credit for it. The day the Prime Minister walks out of No 10 is drawing nearer but, his biographer John Rentoul argues, the manner of his departure will surprise everyone

The Decline and Fall of Tony Blair was always going to offer a fascinating psychological study. He is a talented and lucky person who has suffered unusually few setbacks in life. Not that he is a stranger to adversity. His father's disabling stroke, when Tony was 11; his sister's serious illness which coloured much of his adolescence; and his mother's death from cancer when he was 22 - all impressed on him that life's blessings could be arbitrarily withdrawn. But he himself enjoyed - certainly after he came to terms with a place at only his second-choice Oxford college - what Paddy Ashdown patronisingly called the "long golden upward path".

The Decline and Fall of Tony Blair was always going to offer a fascinating psychological study. He is a talented and lucky person who has suffered unusually few setbacks in life. Not that he is a stranger to adversity. His father's disabling stroke, when Tony was 11; his sister's serious illness which coloured much of his adolescence; and his mother's death from cancer when he was 22 - all impressed on him that life's blessings could be arbitrarily withdrawn. But he himself enjoyed - certainly after he came to terms with a place at only his second-choice Oxford college - what Paddy Ashdown patronisingly called the "long golden upward path".

Entering the legal profession, he not only beat his brainier rival for a place at one of the best-connected chambers, he married her. His failure even to secure Labour's deposit in a by-election in 1982 was no personal humiliation in deep, deep blue Beaconsfield during the apotheosis of Margaret, liberator of the Falklands. On the contrary, it was his introduction to senior Labour politicians, including Michael Foot, the party leader, which made possible the stroke of luck and skill that won him a safe Labour seat the following year.

Entering parliament, he virtually skipped the backbenches and rose through the opposition ranks with the speed and stealth of a shark. The few risks he took paid off in prizes that exceeded his expectations every time. After only 11 years as an MP, when John Smith died, all he had to do was sit tight and try to be nice to Gordon Brown for the Labour leadership to fall into his lap.

More experienced hands smiled indulgently when, as Leader of the Opposition, he excitedly described his mauling by the press over Harriet Harman's choice of a selective school for her son as a foretaste of the storms ahead in government. But in a way he was right and they were wrong.

No incoming government was ever given such a favourable wind by the press. No new prime minister was greeted by such a wave of what Blair called "post-election euphoria" among the electorate.

There was always a defensiveness about New Labour that came from the top, as Blair seemed always to expect his luck to turn. There was a series of crises. The fuel protests, the foot-and-mouth outbreak. But his underlying good fortune - and, of course, sound political strategy - held, and the 2001 election was a mere formality.

But now things have really turned bad. Blair is in what one insider last week called "condition normal" for a British prime minister. He has to worry about getting his legislation through the Commons - turning a majority of 161 into one of five for the vote on variable tuition fees three months ago. He has to face endless speculation about his political longevity. His premiership has been reduced to fire-fighting a series of crises day by day, while many of his attempts to set the tempo of politics end up with him seeming to dance to the tune of others. The decision to hold a referendum on the European constitution is only the most recent example of what might have been hailed as a masterstroke in the early part of his time in office, but is now universally panned as the desperate act of a weak leader.

Coming after 18 months of finding himself for the first time fighting for his reputation - over Iraq - this is without doubt a significant moment. What has changed is the growing realisation that the situation in Iraq is not going to offer a clear-cut vindication. Ever. For much of the past year, Blair has behaved like history's spin doctor, trying to interest the media in the leak of an early draft of posterity's verdict on the Iraq war. Over the past month or so, it has begun to dawn on him that the final report on the war is probably going to come down on the wrong side of "mixed".

So now he is going to have to fight every inch of the way for the right to stay at No 10. Ashdown's golden path has led into a dark, threatening alley - with spikes on the walls, and they are closing in. But it is much too early to assume that, just because Blair is getting deeper into such unfamiliar and difficult territory, he is likely to buckle more easily than a more hardened, Ashdown-like leader.

In particular, the worsening situation in Iraq, hideous though last week's revelations of American and British troops' brutality have been, is unlikely to drive him from office. He has not stood against so much of liberal opinion in this country for so long to give up now.

For all his green naivety in opposition - a fuss about a colleague's choice of school! What wouldn't he give for that to be the worst the press could throw at him now? - Blair has always seemed more sane and balanced than most politicians. They are all egotistical, of course. What makes him different is the extent to which he is able to prevent his vanity interfering with his judgement.

He would have been incensed, for example, by last week's open letter from the 52 former diplomats attacking his policy on Israel and Iraq. But he would have seen instantly that it failed to propose an alternative - the very failure with which he charged Michael Howard as he brushed aside his questions about Iraq in the Commons.

His strength is that he is able to take a detached view of his position, unlike Margaret Thatcher, who, cloistered with sycophants, had no idea how weak she was as she approached her end. He knows that the only way he will be ejected from office against his will is if, as in her case, a clear majority of his MPs demand it. And he is nowhere near that situation yet. He may not be the shining electoral asset to his party that he once was, but he is not yet the liability that she had become a year or 18 months before her fall.

Hence all the speculation of the past week about how long he might continue as Prime Minister turns quite rightly on how long he wants to. And that is unknowable. The front-page lead story in The Times on Thursday was that Blair has told "close cabinet friends" - as opposed to Gordon Brown, presumably, which might have been a more interesting story - that he intends to "serve a full third term if he is re-elected next summer". But, as the political editor noted in the 15th paragraph of the report, that is merely a necessary formula: "Ministers, including Mr Brown, are likely to accept that Mr Blair has no choice but to give the impression that he will go on and on, even if privately that is not his intention."

Indeed, if what Blair is alleged to have told "close cabinet friends" sounds familiar, that might be because it is what the Prime Minister himself confided in Jeremy Paxman in May 2002. For the benefit of those not watching Newsnight, he repeated it in an interview with The Observer in September last year. He even allowed himself to be drawn on a follow-up question. "Does he ever look around the cabinet table and ask himself whether someone else might just conceivably do the job better? He laughs: 'No.'"

The rules of the media game have changed since Harold Wilson's shock resignation in 1976. As the late and sadly missed Ben Pimlott wrote in his great biography, Wilson dropped hints "to anybody who would listen" for three months before he went - including, famously, to James Callaghan in the No 10 lavatory just before the cabinet meeting at which he made his announcement. In fact, he had told Roy Jenkins before the 1970 election that, if he won it, he would stand down during that term. He even mentioned a possible date, 14 June 1973, because then he would have served longer than Asquith, then - until Thatcher - the longest-serving prime minister of the century. In that obsession with historical dates, at least, Blair is like many of his predecessors. Last year, he unwisely anticipated the date on which he would overtake Clement Attlee in length of prime ministerial service. "On 2 August this year we will have been in power for longer than that Attlee government," he told the Fabian Society. By the time the actual date arrived, however, any such grandstanding had been swept aside by the suicide of David Kelly.

Subsequent milestones, namely those marked John Major (overtaken last October), Harold Macmillan (8 February this year) and Ramsay MacDonald (17 February) were passed with rather less fanfare.

But if Blair had a date in mind, which he probably does not, he would not tell anyone except possibly Cherie, Jonathan Powell, his chief of staff, and Sally Morgan, his political adviser. Before the second 1974 election, Wilson told Denis Healey, Tony Benn and Joe Haines, his press secretary, he would go after two years. If Blair did the same, power would drain away from him and to Gordon Brown instantly. That is why Brown can only ever receive undated assurances of Blair's willingness to ease his succession. Those close to both Blair and Brown say that these cloudy promises were renewed at the so-called Granita II dinner at John Prescott's Admiralty Arch apartment last November. But no one should assume that Brown's cheerful demeanour since then means a date has been set.

Blair is said to have given some thought to when and how he might move on, and has even said to some interviewers that he has thought about what he might move on to. His health and the pressures on his family are undoubtedly factors in those decisions. But it has not been possible to detect any slackening in the physical or emotional energy he brings to the job. And his family is important. Cherie finds the demands of her position and the intrusiveness of the press irritating beyond measure. But not beyond endurance. The fact that the Daily Mail news desk begins to get twitchy if the newspaper has not carried a story about her for two days was always a price she was prepared to pay. The fact that she feels she has to pay thousands of pounds out of her own money to take a personal hairdresser with her when she accompanies her husband on international trips is also a price she is prepared, literally, to pay. Although it is said that she resents it.

She may look forward to the day when the daily scrutiny is less intense. But she is proud of her husband and as fiercely determined as he is that his place in history should be recognised.

Blair's departure, then, will be decided not by Gordon Brown or by Cherie or by critics of the Iraq war, or by arbitrary anniversaries. Not his seventh as Prime Minister yesterday, or his 51st birthday on Thursday, or his outlasting Stanley Baldwin in three months' time. He will go when he judges that his reputation will gain more from going than by staying. And he will probably be rather cooler and sharper about that decision than many expect.

John Rentoul's biography 'Tony Blair, Prime Minister' is published by Time Warner Books