Mr Bush told Tony Blair of the extraordinary plan during a meeting in the White House on January 31, 2003, six weeks before the war started, according to an updated version of Lawless World by Philippe Sands, a human rights lawyer. He says the President made it clear that he had already decided to go to war, despite still pressing for a UN resolution.

“The US was thinking of flying U2 reconnaissance aircraft with fighter cover over Iraq, painted in UN colours. If Saddam fired on them, he would be in breach,” the book reports Mr Bush telling Mr Blair at the meeting.

If the U2 idea was a serious proposal, it would have made sense only if the spy plane was ordered to fly at an altitude within range of Iraqi missiles. Mr Bush’s reference in the recorded conversation to the U2 being escorted by fighter aircraft indicates that that is what he had in mind.

The U2, America’s most sophisticated aerial reconnaissance aircraft, can operate at 90,000ft, taking high-resolution photographs of targets. At this altitude, the U2 would have been beyond the range of Iraqi surface-to-air missiles.

U2s were made available to the UN weapons inspectors to help them in their search for Iraq’s weapons of mass destruction (WMD). But there has never been any suggestion until now that Mr Bush had thought about painting one of them in UN colours to deceive Saddam.

Later in the same meeting the President said it was also possible that a defector could be brought out who would give a public presentation about Saddam’s WMD, and there was a small possibility that Saddam would be assassinated.

The book also claims that the President “thought it unlikely that there would be internecine warfare between the different religious and ethnic groups”.

President Bush also made clear, according to the book which was featured on Channel 4 News, that he would go to war irrespective of whether there was a second UN resolution.

“The US would put its full weight behind efforts to get another resolution and would ‘twist arms’ and ‘even threaten’. But he had to say that if, ultimately, we failed, military action would follow anyway,” the book said.

The section of the book is based on a memo of the meeting. Mr Blair responded that he was “solidly with the President and ready to do whatever it took to disarm Saddam”. But the Prime Minister said that a second Security Council resolution would provide an insurance policy against the unexpected, and international cover, including with the Arabs.

Mr Sands’ book says that the meeting focused on the need to identify evidence that Saddam had committed a material breach of his obligations under the existing UN Resolution 1441. There was concern that insufficient evidence had been unearthed by the UN inspection team, led by Dr Hans Blix.

That was why other options, such as the aircraft in UN colours, were considered.

Last night Sir Menzies Campbell, acting Liberal Democrat leader, said: “If these allegations are accurate, the Prime Minister and President Bush were determined to go to war with or without a second UN resolution, and Britain was signed up to do so by the end of January 2003.”

He added: “By then it was clear that there was no credible evidence of weapons of mass destruction, the stated justification for the moves against Saddam Hussein. The fact that consideration was apparently given to using American military aircraft in UN colours to provoke Saddam graphically illustrates the rush to war.”

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