A new book exposes how the two men connived to dupe the United Nations and blows the lid off Mr Blair's claim that he was a restraining influence on Mr Bush.

He offered his total support for the war at a secret White House summit as Mr Bush displayed his contempt for the UN, made a series of wild threats against Saddam Hussein and showed a devastating ignorance about the catastrophic aftermath of the war.

Based on access to information at the highest level, the book by leading British human rights lawyer Philippe Sands QC, Professor of Law at London University, demonstrates how the two men decided to go to war regardless of whether they obtained UN backing.

The revelations make a nonsense of Mr Blair's claim that the final decision was not made until MPs voted in the Commons 24 hours before the war - and could revive the risk of him being charged with war crimes or impeached by Parliament itself.

The book also makes serious allegations concerning the conduct of Foreign Secretary Jack Straw, Lord Chancellor Lord Falconer and Attorney General Lord Goldsmith over Goldsmith's legal advice on the war.

And it alleges the British Government boasted that disgraced newspaper tycoon Conrad Black was being used by Mr Bush's allies in America as a channel for pro-war propaganda in the UK via his Daily Telegraph newspaper.

The leaks are contained in a new version of Sands' book Lawless World, first published last year, when it emerged that Lord Goldsmith had told Mr Blair the war could be unlawful - before a lastminute U-turn.

The new edition, to be published by Penguin on Thursday, is likely to cause a fierce new controversy on both sides of the Atlantic.

It follows recent charges against two British men under the Official Secrets Act after a transcript of another conversation between Mr Bush and Blair, in which the President raised the possibility of bombing the Al Jazeera Arab TV station, was leaked by a Whitehall official.

Both governments will be horrified that the stream of leaks revealing the grim truth about the war is turning into a flood. The most damaging new revelation concerns the meeting between Mr Blair and Mr Bush at the White House on January 31, 2003, during which Mr Blair urged the President to seek a second UN resolution giving specific backing for the war.

The Mail on Sunday has established that the meeting was attended only by Mr Blair, his Downing Street foreign policy adviser Sir David Manning, Mr Bush and the President's then national security adviser Condoleezza Rice, plus an official note-taker.

The top-secret record of the meeting was circulated to a tiny handful of senior figures in the two administrations.

Immediately afterwards, the two leaders gave a Press conference in which a nervous-looking Mr Blair claimed the meeting had been a success. Mr Bush gave qualified support for going down the UN route. But observers noted the awkward body language between the two men. Sands' book explains why. Far from giving a genuine endorsement to Mr Blair's attempt to gain full UN approval, Mr Bush was only going through the motions. And Mr Blair not only knew it, but went along with it.

The description of the January 31 meeting echoes the recent memoirs of Britain's former ambassador to Washington, Sir Christopher Meyer.

Meyer, who was excluded from the private session between Blair and Bush, claimed the summit marked the culmination of the Prime Minister's failure to use his influence to hold back Mr Bush.

Equally significantly, Meyer was puzzled by Blair's behaviour when the two leaders emerged to join other aides. Meyer writes: "We were all milling around in the State dining room as Bush and Blair put the final touches to what they were going to say to the media.

"Bush had a notepad on which he had written a form of words on the second resolution...He read it out...There was silence. I waited for Blair to say he needed something as supportive as possible. He said nothing. I waited for somebody on the No 10 team to say something. Nothing was said. I cursed myself afterwards for not piping up.

"At the Press conference, Bush gave only a perfunctory and lukewarm support for a second resolution. It was neither his nor Blair's finest performance."

In view of Sands' disclosures, Blair had every reason to look awkward: he knew that despite his public talk of getting UN support, privately he had just committed himself to going to war no matter what the UN did.

When, in due course, the UN refused to back the war, Mr Blair seized on the fact that French President Jacques Chirac said he would not support any pro-war resolution, claiming that the French veto was so 'unreasonable' that a UN vote was pointless. In reality, Bush and Blair had decided to go to war before Chirac uttered a word.

The disclosures will be seized on by anti-war critics in Britain, including Left-wing MPs who say Mr Blair should be impeached for his handling of the war.

However, Ministers will argue that after three major British inquiries into the war, and with thousands of British troops due to be sent home from Iraq this year, it is time to move on.

A Downing Street spokeswoman said last night: "These matters have been thoroughly investigated and we stand by our position."