The former prime minister has always defended the legality of the war as being based on the argument that both the UK and the US believed the threat from Saddam’s WMD contravened United Nations resolutions.

However, his revelation, in a BBC interview to be broadcast today, that he would have removed the Iraq leader whether he had WMD or not, and “deployed different arguments” to justify military action, will re-ignite the debate that Mr Blair lied to Commons ahead of the invasion in 2003.

In the BBC programme “Fern Britton Meets …Tony Blair” the former PM speaks about Saddam and says his threat to the wider Middle East region was enough. “I would still have thought it right to remove him,” he says.

“Obviously you would have had to use and deploy different arguments about the nature of the threat.”

Mr Blair describes the development of Saddam’s WMD programme as “obviously one” of the threats.

The Iraq Inquiry headed by Sir John Chilcott is scheduled to take evidence from Mr Blair early next year. His comments to the BBC now puts centre stage the issue of what the former prime minister knew in advance of the invasion.

The inquiry recently took evidence from Sir Jeremy Greenstock, the UK’s UN ambassador in the run-up to the Iraq invasion. Sir Jeremy told the inquiry that the invasion was “because of their [Iraq] contravening of UN declarations on weapons of mass destruction, and nothing else.”

Mr Blair’s comments that other arguments to invade would have been deployed are in direct conflict with Greenstock and others on the issue of WMD. The former ambassador said he regarded the military action as being legal but of “questionable legitimacy”.

Britton is told by Mr Blair that he did not think Iraq would be better off if Saddam and his two sons were still in charge. He says: “I sympathise with the people who were against it for perfectly good reason and are against it now, but for me, in the end, I had to take the decision.” He later says: “You’ve got to work out what is right.”

During a debate in the Commons on March 18, 2003, the former foreign secretary Robin Cook said the issue of Iraq’s WMD had not been resolved by the UN’s weapons inspectors.

Cook later gave an interview in which he said Mr Blair knew at the time of the Commons debate that the claims of WMD in Iraq were not true. He believed that if the Government had been forced to withdraw the dossier it had drawn up in late 2002 as evidence of Saddam’s WMD programme, Mr Blair’s position would have been untenable.

Cook also believed that if Mr Blair had revealed any doubts about Iraq’s WMD or offered other arguments to justify the military action, he would have made it impossible for the then attorney general, Lord Goldsmith, to offer full legal cover for the decision.

On the eve of the invasion, the head of the Army asked Downing for assurances that the war was legal.

In the BBC interview, Mr Blair acknowledges there are families who blame him for the deaths of their loved ones. He tells Britton: “That’s the responsibility you carry. But … there’s no point in going into a situation of conflict and not understanding there is going to be a price to be paid.”

The former Liberal Democrat leader Sir Menzies Campbell said that Mr Blair would not have obtained the support of the Cabinet or Parliament for war if he had been so open about his view on regime change at the time.

“In spite of experience, in spite of the benefit of hindsight, Mr Blair still does not realise just how much of a foreign policy disaster Britain’s involvement in the military action against Iraq turned out to be,” he told the BBC Radio 4 Today programme.

“I have no doubt whatsoever that if Mr Blair had told his Cabinet what he is now saying, he’d have found it very difficult to keep all of them - he did of course lose Robin Cook and eventually Clare Short.

“But the one place he would have undoubtedly failed would have been in the House of Commons. He would not have obtained the endorsement of the House of Commons on March 18, 2003 if he had been as frank with the House of Commons then as he appears to be willing to be frank with the BBC now.”

Angus Robertson MP, SNP Westminster leader and Defence spokesman said: “Tony Blair appears to be re-writing history before he is called before the Iraq inquiry, but his admission now raises crunch questions for the current prime minister. Would Gordon Brown still have bankrolled the war had he known there were never any weapons of mass destruction in Iraq?

“The heat is now on Gordon Brown, as the Chancellor who wrote the cheques for this disastrous and illegal war.

“Tony Blair took us to war on a false prospectus, and Gordon Brown bankrolled it - both men should appear side by side when they give evidence, so that we can get to the truth behind the biggest foreign policy disaster in modern times.

“This inquiry will be judged on the answers that it provides, and these fundamental questions must be addressed.”