Tony Blair opened himself up to a charge of misleading Parliament today when he told the Iraq inquiry that by any objective analysis the threat posed by Saddam Hussein's chemical and biological weapons programme had not increased after 9/11.

Appearing as the star witness before Sir John Chilcot's panel, the former Prime Minister stressed the transformation of US and UK foreign policy after the attacks of September 11, 2001, an event that he said had changed the "calculus of risk" for the transatlantic allies.

However, as part of that analysis Mr Blair conceded that the threat posed by Saddam Hussein's purported programme to develop weapons of mass destruction (WMD) had not actually grown — only the understanding of that threat.

“It wasn’t that objectively he had done more," he said of the Iraqi leader. "It was that our perception of the risk had shifted." The comment appeared to contradict a statement that Mr Blair made to the Commons on September 24, 2002, as he presented what later became known as the "sexed-up" dossier on Iraq's WMD programme. That dossier included the now discredited claim that President Saddam could deploy WMD within 45 minutes.

Mr Blair told MPs: “His weapons of mass destruction programme is active, detailed and growing. The policy of containment is not working. The weapons of mass destruction programme is not shut down; it is up and running now.”

Several hundred demonstrators — chanting "Jail Tony" and "Blair lied" — gathered outside the Queen Elizabeth II conference centre, although the former prime minister managed to slip in via a cordoned-off back entrance two hours before he was due to appear.

It was a far cry from the mass marches of February 2003, a month before the invasion, when millions marched around the country to protest against the looming war.

In those days Mr Blair was routinely derided as "George Bush's poodle", but today he denied that he had ever made any private deals with the US President before the invasion of Iraq.

Instead, he said, he had always made clear that he would have to join the Americans if it came to military action to overthrow Saddam.

"This is an alliance that we have with the United States of America," he said. "It is not a contract; it's not, 'You do this and we'll do that'." Mr Blair confirmed that he had discussed the issue of Iraq when he met Mr Bush for private, one-to-one talks at his Texas ranch at Crawford in April 2002, 11 months before the invasion, but he insisted that they did not get into “specifics”.

“What I was saying — I was not saying this privately, incidentally, I was saying it in public — was, ‘We are going to be with you in confronting and dealing with this threat’.

“The one thing I was not doing was dissembling in that position. How we proceed in this is a matter that was open. The position was not a covert position, it was an open position. We would be with them in dealing with this threat and how we did that was an open question, and even at that stage I was raising the issue of going to the UN.”