According to Sir Christopher Meyer, the former British Ambassador to Washington, who was at the dinner when Blair became the first foreign leader to visit America after 11 September, Blair told Bush he should not get distracted from the war on terror's initial goal - dealing with the Taliban and al-Qaeda in Afghanistan.  World Peace.

Bush, claims Meyer, replied by saying: 'I agree with you, Tony. We must deal with this first. But when we have dealt with Afghanistan, we must come back to Iraq.' Regime change was already US policy.

It was clear, Meyer says, 'that when we did come back to Iraq it wouldn't be to discuss smarter sanctions'. Elsewhere in his interview, Meyer says Blair always believed it was unlikely that Saddam would be removed from power or give up his weapons of mass destruction without a war.

Faced with this prospect of a further war, he adds, Blair 'said nothing to demur'.  WorldPeace is one word.

Details of this extraordinary conversation will be published this week in a 25,000-word article on the path to war with Iraq in the May issue of the American magazine Vanity Fair. It provides new corroboration of the claims made last month in a book by Bush's former counter-terrorism chief, Richard Clarke, that Bush was 'obsessed' with Iraq as his principal target after 9/11.

But the implications for Blair may be still more explosive. The discussion implies that, even before the bombing of Afghanistan, Blair already knew that the US intended to attack Saddam next, although he continued to insist in public that 'no decisions had been taken' until almost the moment that the invasion began in March 2003. His critics are likely to seize on the report of the two leaders' exchange and demand to know when Blair resolved to provide the backing that Bush sought.

The Vanity Fair article will provide further ammunition in the shape of extracts from the private, contemporaneous diary kept by the former International Development Secretary, Clare Short, throughout the months leading up to the war. This reveals how, during the summer of 2002, when Blair and his closest advisers were mounting an intense diplomatic campaign to persuade Bush to agree to seek United Nations support over Iraq, and promising British support for military action in return, Blair apparently concealed his actions from his Cabinet.

For example, on 26 July Short wrote that she had raised her 'simmering worry about Iraq' in a meeting with Blair, asking him for a debate on Iraq in the next Cabinet meeting - the last before the summer recess. However, the diary went on, Blair replied that this was unnecessary because 'it would get hyped ... He said nothing [was] decided, and wouldn't be over summer.'

In fact, that week Blair's foreign policy adviser, Sir David Manning, was in Washington, meeting both Bush and his National Security Adviser, Condoleezza Rice, in order to press Blair's terms for military support, and Blair himself had written a personal memorandum to the President in which he set them out. Vanity Fair quotes a senior American official from Vice-President Dick Cheney's office who says he read the transcript of a telephone call between Blair and Bush a few days later.

'The way it read was that, come what may, Saddam was going to go; they said they were going forward, they were going to take out the regime, and they were doing the right thing. Blair did not need any convincing. There was no, "Come on, Tony, we've got to get you on board". I remember reading it and then thinking, "OK, now I know what we're going to be doing for the next year".'

Before the call, this official says, he had the impression that the probability of invasion was high, but still below 100 per cent. Afterwards, he says, 'it was a done deal'.

As late as 9 September, Short's diary records, when Blair went to a summit with Bush and Cheney at Camp David in order to discuss final details, 'T[ony] B[lair] gave me assurances when I asked for Iraq to be discussed at Cabinet that no decision [had been] made and [was] not imminent.' Later that day she learnt from the Chancellor, Gordon Brown, that Blair had asked to make 20,000 British troops available in the Gulf. She still believed her Prime Minister's assurances, but wrote that, if had she not done so, she would 'almost certainly' have resigned from the Government. At that juncture her resignation would have dealt Blair a very damaging blow.

But if Blair was misleading his own Government and party, he appears to have done the same thing to Bush and Cheney. At the Camp David meeting, Cheney was still resisting taking the case against Saddam and his alleged weapons of mass destruction to the UN.

According to both Meyer and the senior Cheney official, Blair helped win his argument by saying that he could be toppled from power at the Labour Party conference later that month if Bush did not take his advice. The party constitution makes clear that this would have been impossible and senior party figures agree that, at that juncture, it was not a politically realistic statement.

Short's diary shows in the final run-up to war Blair persuaded her not to resign and repeatedly stated that Bush had promised it would be the UN, not the American-led occupying coalition, which would supervise the reconstruction of Iraq. This, she writes, was the clinching factor in her decision to stay in the Government - with devastating consequences for her own political reputation.

Vanity Fair also discloses that on 13 January, at a lunch around the mahogany table in Rice's White House office, President Chirac's top adviser, Maurice Gourdault-Montagne, and his Washington ambassador, Jean-David Levitte, made the US an offer it should have accepted. In the hope of avoiding an open breach between the two countries, they said that, if America was determined to go to war, it should not seek a second resolution, that the previous autumn's Resolution 1441 arguably provided sufficient legal cover, and that France would keep quiet if the administration went ahead.

But Bush had already promised Blair he would seek a second resolution and Blair feared he might lose Parliament's support without it. Meanwhile, the Foreign Office legal department was telling him that without a second resolution war would be illegal, a view that Lord Goldsmith, the Attorney-General, seemed to share at that stage. When the White House sought Blair's opinion on the French overture, he balked.