Sir Christopher claimed in his outspoken memoirs that Britain's support for military action was "taken for granted" by the White House, after Mr Blair agreed to military action with reservations at the Bush ranch in Crawford, Texas, in April 2002.
At his monthly press conference yesterday, the Prime Minister responded with claims that the French President, Jacques Chirac, was to blame for the slide to war without a second UN resolution. He said the French had threatened to veto a second UN resolution on Iraq.
Mr Blair told journalists: "If you go back and look at what happened in March 2003, I think you will see that I made the most strenuous efforts to get a second UN resolution and to end up with a second resolution that would have given us more time. The fact is, we couldn't get one for a very simple reason: the French made it clear they would veto any such resolution."
Senior French sources accused Mr Blair of a faulty memory. "Only four out of the 15 members of the Security Council supported a new resolution and Britain needed nine to win approval. It is completely wrong to blame it on France," said one official.
Sir Christopher supported the French in concluding that the French opposition to the draft UN resolution was not final. He said: "I never interpreted the French refusal to accept the draft of a second resolution as a refusal for ever and a day. In diplomacy, you never say never.
"Talking to me in private, French officials accuse America and Britain of deliberately exaggerating France's position to justify going to war without further UN cover."
Sir Christopher, 61, a career diplomat, said that in early October 2002 he asked a White House contact whether the US mobilisation for war had advanced so far that it was unstoppable. "I was told that the President had not yet signed off on going to war. Nothing was irrevocable."
He said President Bush would still have faced an agonising decision if he had encountered opposition from his key ally, Britain, in the weeks before the war in 2003. "The advice the British Prime Minister then gave the US President would never have been more important in my time in Washington. It could even be the swing vote for war or peace. The pendulum never swung back again."
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