Blair's litany of failures on Iraq - ambassador's damning verdict

He says Mr Blair failed to exploit his enormous leverage with Mr Bush not only to secure a precious delay but to plan for postwar Iraq. "We may have been the junior partner in the enterprise but the ace up our sleeve was that America did not want to go it alone. Had Britain so insisted, Iraq after Saddam might have avoided the violence that may yet prove fatal to the entire enterprise."

But Mr Blair did not have any appetite for bargaining with Mr Bush, according to Sir Christopher: "Tony Blair chose to take his stand against Saddam and alongside President Bush from the highest of high moral ground. It is the definitive riposte to Blair the Poodle, seduced though he and his team always appeared to be by the proximity and glamour of American power.

"But the high moral ground, and the pure white flame of unconditional support to an ally in service of an idea, have their disadvantages. They place your destiny in the hands of an ally. They fly above the tangled history of Sunni, Shia, Kurd, Turkomen and Assyrian. They discourage descent into the dull detail of tough and necessary bargaining: meat and drink to Margaret Thatcher but, so it seemed, uncongenial to Tony Blair."

The former diplomat accuses Mr Blair of weakness in failing to engage Mr Bush in the "plain-speaking conversation" that needed to take place. "Had Blair told Bush in clear and explicit terms that he would be unable to support a war unless British wishes were met? I doubted it."

The Washington embassy repeatedly advised Downing Street to use its leverage, but was ignored.

Delaying the invasion from March to the autumn would have allowed the United Nations weapons inspectors extra months to establish whether Saddam Hussein had weapons of mass destruction, enabled the US and Britain to reach an understanding with France and Russia, two of the biggest sceptics about war, and increased international support, instead of going to war "in the company of a motley ad hoc coalition of allies".

The former diplomat, who enjoyed unparalleled access to all the key members of Mr Bush's administration and supported the war, provides the most detailed account yet of the thinking inside the White House and Downing Street in the 18 months running up to the invasion in March 2003. He says of the war now: "History's verdict looks likely to be that it was terminally flawed both in conception and execution."

Publication comes at a time when Mr Blair is vulnerable domestically, and the indictment of Lewis "Scooter" Libby, the chief of staff of Vice-President Dick Cheney, has reopened the debate in the US about why the country went to war.

Sir Christopher records a conversation with Mr Libby who told him "we were the only ally that mattered. That was a powerful lever". But the former ambassador says London "was not fertile ground for the notion of leverage or the tough negotiating position that must sometimes be taken even with the closest allies - as Churchill did with Roosevelt and Thatcher did with Reagan".

He regrets that at precisely the moment that Mr Blair should have been bargaining, in the early autumn of 2002, "political energy in London had become consumed by a titanic struggle to keep public opinion, parliament and the Labour party onside for war. There was little energy left in No 10 to think about the aftermath. Since Downing Street drove Iraq policy, efforts made by the Foreign Office to engage with the Americans on the subject came to nothing."

He questions whether No 10 relied too heavily on British military and intelligence advisers fatalistic about the inevitability of war and "as a consequence underestimated its political leverage and ability to affect the course of events".

He takes a swipe at John Scarlett, chairman of the joint intelligence committee, which was responsible for assessing intelligence, and one of the main authors of the controversial British dossier making a case against Saddam. Sir Christopher, who at one time was lined up to be head of the JIC, said he understood why Alastair Campbell, Mr Blair's press secretary, wanted as categorical a public depiction of Saddam's threat as possible. "Equally, I would have expected the JIC to be rigorous in telling me how far I could go."

One of Sir Christopher's main charges is that Mr Blair failed to puncture the US administration's belief that it would be "sweetness and light in Iraq" after the war, and the descent of Iraq into chaos today is, in part, a result of this.

Sir Christopher recounts how Mr Bush told the inner circle at a US-British summit at Camp David in 2002 that the prime minister had "cojones" (balls). The former ambassador says Britain should have taken advantage of such praise, making its participation in the war dependent on a fully worked-out plan for postwar Iraq, which he describes as "defective" and "rudimentary".

"This would have been the appropriate quid pro quo for Blair's display of cojones at this Camp David meeting with Bush." He is adamant Mr Bush was amenable to pressure almost to the end. "Indeed, if it all went wrong at the UN, and the US was faced with going to war alone, it seemed to me that Bush might blink. Or, to put it another way: what Britain decided to do could be the decisive factor in the White House."

The former ambassador says a delay from March to autumn 2003 could have made a significant difference: "Even if the most optimistic predictions are finally realised for Iraq, the question will still be asked: why did the Americans and British make it so hard for themselves and even harder for Iraqis? The US and the UK would have stood a better chance of going to war in good order, and of doing the aftermath right, had they planned on an autumn, not a spring, campaign."

He reveals that Karl Rove, the political adviser to the president, told him there would have been no problem for Mr Bush in waiting until the end of 2003 or even early 2004 and this would not have risked entanglement in the US presidential campaign.