The warning, in a leaked Cabinet Office briefing paper, said Tony Blair had already agreed to back military action to get rid of Saddam Hussein at a summit at the Texas ranch of President George W Bush three months earlier.

The briefing paper, for participants at a meeting of Blair’s inner circle on July 23, 2002, said that since regime change was illegal it was “necessary to create the conditions” which would make it legal.

This was required because, even if ministers decided Britain should not take part in an invasion, the American military would be using British bases. This would automatically make Britain complicit in any illegal US action.

“US plans assume, as a minimum, the use of British bases in Cyprus and Diego Garcia,” the briefing paper warned. This meant that issues of legality “would arise virtually whatever option ministers choose with regard to UK participation”.

The paper was circulated to those present at the meeting, among whom were Blair, Geoff Hoon, then defence secretary, Jack Straw, the foreign secretary, and Sir Richard Dearlove, then chief of MI6. The full minutes of the meeting were published last month in The Sunday Times.

The document said the only way the allies could justify military action was to place Saddam Hussein in a position where he ignored or rejected a United Nations ultimatum ordering him to co-operate with the weapons inspectors. But it warned this would be difficult.

“It is just possible that an ultimatum could be cast in terms which Saddam would reject,” the document says. But if he accepted it and did not attack the allies, they would be “most unlikely” to obtain the legal justification they needed.

The suggestions that the allies use the UN to justify war contradicts claims by Blair and Bush, repeated during their Washington summit last week, that they turned to the UN in order to avoid having to go to war. The attack on Iraq finally began in March 2003.

The briefing paper is certain to add to the pressure, particularly on the American president, because of the damaging revelation that Bush and Blair agreed on regime change in April 2002 and then looked for a way to justify it.

There has been a growing storm of protest in America, created by last month’s publication of the minutes in The Sunday Times. A host of citizens, including many internet bloggers, have demanded to know why the Downing Street memo (often shortened to “the DSM” on websites) has been largely ignored by the US mainstream media.

The White House has declined to respond to a letter from 89 Democratic congressmen asking if it was true — as Dearlove told the July meeting — that “the intelligence and facts were being fixed around the policy” in Washington.

The Downing Street memo burst into the mainstream American media only last week after it was raised at a joint Bush-Blair press conference, forcing the prime minister to insist that “the facts were not fixed in any shape or form at all”.

John Conyers, the Democratic congressman who drafted the letter to Bush, has now written to Dearlove asking him to say whether or not it was accurate that he believed the intelligence was being “fixed” around the policy. He also asked the former MI6 chief precisely when Bush and Blair had agreed to invade Iraq and whether it is true they agreed to “manufacture” the UN ultimatum in order to justify the war.

He and other Democratic congressmen plan to hold their own inquiry this Thursday with witnesses including Joe Wilson, the American former ambassador who went to Niger to investigate claims that Iraq was seeking to buy uranium ore for its nuclear weapons programme.

Frustrated at the refusal by the White House to respond to their letter, the congressmen have set up a website — www.downingstreetmemo.com — to collect signatures on a petition demanding the same answers.

Conyers promised to deliver it to Bush once it reached 250,000 signatures. By Friday morning it already had more than 500,000 with as many as 1m expected to have been obtained when he delivers it to the White House on Thursday.

AfterDowningStreet.org, another website set up as a result of the memo, is calling for a congressional committee to consider whether Bush’s actions as depicted in the memo constitute grounds for impeachment.

It has been flooded with visits from people angry at what they see as media self-censorship in ignoring the memo. It claims to have attracted more than 1m hits a day.

Democrats.com, another website, even offered $1,000 (about £550) to any journalist who quizzed Bush about the memo’s contents, although the Reuters reporter who asked the question last Tuesday was not aware of the reward and has no intention of claiming it.

The complaints of media self-censorship have been backed up by the ombudsmen of The Washington Post, The New York Times and National Public Radio, who have questioned the lack of attention the minutes have received from their organisations.