Presume not that I am the thing I was,
For God doth know, so shall the world perceive,
That I have turned away my former self;
So will I those that kept me company.
Henry V (Henry IV Part Two, Act V, Scene 5)

If seven days is a long time in politics, seven years is positively an eternity. And the distance that Tony Blair has travelled in that time (not just back and forth across the Atlantic) remains a striking feature of any current assessment. True, we were told at the outset that having won as new Labour, he would govern as new Labour; but how have we got to the situation where the election of a socialist government in Spain and the (slim?) possibility of a Democratic victory in America can cause our own Prime Minister such embarrassment? After all, a Labour prime minister traditionally supports the Democratic candidate: but then, a Labour prime minister traditionally supports the Labour Party.

In 1997, Blair could count among his friends and admirers the likes of Jacques Chirac, Gerhard Schroder, Bill Clinton and Nelson Mandela. These days it is George W Bush, Vladimir Putin and Silvio Berlusconi. For New Blair, France, Germany and even the United Nations are the Paula Radcliffes of international politics: when the heat was really on, they failed to go the extra mile.

What frightens me is that, having gone away for the summer and had a chance to reflect on the past year, the Prime Minister has returned more brazenly sure of himself, more self-deluded than ever. It is no surprise that the man behind the FCUK advertising campaign was appointed to spearhead Labour's 2001 publicity drive: Blair himself has entered the "FCUK YOU" phase of his premiership.

It's the closeness to Berlusconi that is most extraordinary, and revealing. Granted, the two meet often on business: one of their encounters was in March, when they hatched a plan to lift the EU arms embargo on Libya so that our two countries could be the first to reward Muammar Gaddafi for giving up his nuclear weapons - by selling him conventional ones. Yet this was different: this was for pleasure. I have a picture in my mind of Tony Blair sitting beside a pool surrounded by olive groves, watching Berlusconi's yacht bobbing on the Mediterranean and wondering how to cut incapacity benefit.

This government has always been driven by Blair's personal obsessions - by the passion of whatever it is that happens to seize him at any given moment. Those who complain that critics go for the man and not the ball miss the point: this is a man who doesn't pass the ball. Time and again, he reduces the whole of government to his own persona. Conference-goers will remember his "irreducible core" speech of 2000. "Labour, c'est moi," was the inescapable message. This reached its apotheosis during the Hutton inquiry, when Blair said that the BBC allegations against him "went, in a sense, to the credibility, I felt, of the country". To question his integrity is to question that of Britain itself.

Do we really have to put up with such patent nonsense? There is nothing wrong with strong vision and firm leadership; what is worrying is where that vision is based not on fact and objective judgement, but on belief alone. "I may be wrong about this," Blair told Newsnight's Jeremy Paxman on the eve of the invasion of Iraq, "but it's what I believe." He may tell you that black is white, but, for him at least, that can never be a lie because at the time he says it he honestly, sincerely believes that black is white. What we are witnessing is the advent of a faith-based premiership, one which leaves the Prime Minister open to the accusation that his policies may be the product of "an unchecked and unbalanced mind . . . [that] . . . came to confuse the notion of knowing your own mind with refusing to listen to anyone else". He may recognise the words: they are his own description of Margaret Thatcher.

For a recent show at Shakespeare's Globe Theatre, I was encouraged to explore the parallels between the leaders in his plays and those of our own time. The New York Times writer Nicholas Kristof had recently been thinking on similar lines, observing that Shakespeare's world was, like our own, "full of nuances and uncertainties" and that Shakespeare's leaders "self-destruct when they are too rigid, too sure of themselves, or too intoxicated by moral clarity". The quotation at the top of this piece, in which the young Henry V casts aside his erstwhile supporters, seems strikingly pertinent to Blair's current, dangerously hubristic state.

Perhaps it was ever thus. Paradoxically for one supposedly so eager to please every- body, Blair has always revelled in taking on long-standing or apparently impossible arguments and winning people over. It appeals to the barrister in him. He did it with Clause Four. When the chips are down (foundation hospitals, tuition fees, Iraq) he chooses not to side with the Labour Party but to tackle it head-on.

The journey from Blair to here is one that has fascinated me over the past ten years. It has taken me from believer to agnostic to wondering now whether there is a Blair at all - and if there is, what sort of Blair he is. The evidence is that he is a very different man, in policy terms, from the one who assumed the leadership back in 1994. Even in the past two years, we have gone from conciliatory at the 2002 conference ("the test is to listen, adapt and move forward") to defiant in 2003 ("I've not got a reverse gear").

The contrast with the Blair of the late 1980s and early 1990s is even more dramatic: then, he was passionately opposed to electricity privatisation ("We will speak up for a country that knows the good sense of a public industry in public hands") and private prisons ("Privatisation is not just a diversion from the agenda of prison reform; it is fundamentally flawed in principle"). Now, freed from the responsibilities of opposition, his government has embraced entirely the principle of private sector management. On issue after issue, the record shows that, having made a series of statements of policy and ideology in the 1990s, he came to power only to adopt the very policies he had attacked on his way there, albeit with some adaptation.

He explained the philosophy behind this to the Fabian Society in June last year: "We are attempting to take traditional values - equality, liberty, solidarity, democracy, justice - but find modern means to give them expression." He wanted to give these concepts new meaning. And to be fair, he has. He has given them a completely new meaning. He talks of equality, yet the gap between rich and poor has widened. He talks of solidarity, but defies both the Labour Party and the United Nations.

For me, the "forces of conservatism" speech of 1999 was a critical moment in Blair's premiership. Up until that point he could plausibly claim to lead a one-nation government. But, having no enemies to speak of, he chose to create them overnight - with the help of Peter Mandelson, whose idea the speech supposedly was. While the speech was intended in part to challenge vested interests in his own party, its attack on conservatism would show Blair's Labour credentials - much like this month's ban on fox-hunting. Like the ban, however, it merely succeeded in igniting fury and dividing the country.

Our early TV portrayals of Blair painted a picture of an eager-to-please prime minister in thrall to the findings of focus groups and the Daily Mail. Iraq, with its apparent moral imperatives, was the crucible (to use the current metaphor) in which a new Blair was forged: one where nothing counts for so much as his own self-belief. It is a dangerous development. You are either for him or against him (the only backbencher - just the one - promoted in this month's reshuffle was the ultra-loyalist Tom Watson). Blair may have calculated that he might as well be hung for a sheep as a lamb; but with each passing day, in Baghdad or Westminster, the noose is beginning to tighten.

Bremner, Bird and Fortune returns to Channel 4 this Sunday. You Are Here is published by Weidenfeld & Nicolson on 4 November