Professor Philippe Sands QC, director of the Centre of International Courts and Tribunals at University College London, and a member of Cherie Blair’s Matrix law chambers, said the former prime minister’s admission that he would have deployed “different arguments” besides the weapons to justify the war and the removal of Saddam, means “he fixed on the policy first and then found the justification”.

Prof Sands, who claims Mr Blair and the former US president George Bush violated international law in the 2003 invasion, said: “The fact that the policy was fixed by Tony Blair irrespective of the facts on the ground, and irrespective of the legality, will now expose him more rather than less to legal difficulties.”

Mr Blair had, until an interview with the BBC to be broadcast today, maintained he waited until almost the eve of the March 2003 invasion before making a final decision to commit British forces to war in Iraq. Only weeks before the invasion, he stated that if Saddam handed over his stock of WMD, there would be no war. ­Critics of the war’s legality, such as Prof Sands, now believe Mr Blair can no longer sustain this assurance.

Hans Blix, who headed the UN inspection team prior to the invasion, said there was a “lack of sincerity” in Mr Blair’s statement.

Mr Blix said the war was “sold” on the issue of WMD, which were not found after the invasion.

He said: “Now we hear it was only a question of deployment of arguments. It sounds a bit like a fig leaf was held up and if that didn’t work they’d put another fig leaf there.”

One of the immediate legal problems for Mr Blair will include his appearance before the Iraq Inquiry early next year.

The inquiry team headed by Sir John Chilcott already has a number of legal documents about the war which have not been disclosed in the public sessions.

Among them is a letter from the then attorney general, Lord Goldsmith, sent in July 2002 to Mr Blair, informing him that deposing Saddam Hussein by force would breach international law.

At that time Mr Blair had already held private discussions with George Bush on “regime change” in Iraq.

Goldsmith’s written advice is believed to point out that military intervention on the grounds of self defence did not apply as the UK was under no imminent threat from Iraq; that humanitarian intervention was irrelevant; and that earlier United Nations resolutions on Saddam did not hold a legal authority beyond the UN itself.

Lord Goldsmith continued to question the legality of the invasion throughout 2002 and early 2003. This eventually led Number 10, close to the invasion date, to seek backing from the international law professor at the London School of Economics, Christopher Greenwood, one of the only lawyers in the UK who felt long-standing UN resolutions against Iraq made invasion legal.

Only a few members of Mr Blair’s so-called war cabinet are said to have known of Goldsmith’s doubts.

However Mr Blair’s confession to the BBC’s Fern Britton – where he says “I would still have thought it right to remove him. Obviously you would have had to use and deploy different arguments, about the nature of the threat” – means his concern for international law effectively played no part in his decision to send UK forces against Saddam.

For Mr Blair’s critics, there is suspicion that he understands the importance the Goldsmith letter now holds, and how much it can damage his reputation. Prof Sands, however, believes Mr Blair may only have increased the difficulties he faces in front of the inquiry and beyond, by choosing to effectively confess now.

Had Mr Blair been as dismissive about the importance he gave to WMD in the Commons during the pre-invasion debate on Iraq, many MPs now doubt the government would have won. Had he lost, Mr Blair has privately told his closest colleagues he would have immediately resigned as prime minister.

Sir Menzies Campbell said that Mr Blair would not have obtained the support of parliament for the war if he had been so open about his views on the right of regime change.

He added: “I have no doubt whatsoever that if Blair had told his cabinet what he’s saying now, he would have found it difficult to keep all of them.”

Angus Robertson, leader of the SNP in Westminster, said Mr Blair was trying to “rewrite history” ahead of appearing before the Iraq Inquiry.