Tony Blair has begun giving evidence at the Iraq Inquiry - and has said that the UK's policy towards Saddam was to contain the risk posed by the Iraqi leader.

The former Prime Minister is currently being grilled about why he took Britain to war in March 2003 despite massive public opposition and serious questions about the legality of the conflict.

Before Mr Blair started giving his evidence, the inquiry's chairman Sir John Chilcot told him he would be required to confirm that he had told the truth to the inquiry.

He said: "I remind every witness that he will later be asked to sign a transcript of the evidence to the effect that the evidence given is truthful, fair and accurate."

Sir John said Friday's session would focus on three key questions - "Why really did we invade Iraq, why Saddam, and why now in March 2003?"

Opening his evidence, Mr Blair emphasised the impact that the 9/11 attacks had on his thinking and the thinking in the US.

"Up to September 11 we thought he was a risk but we thought it was worth trying to contain it. Crucially, after September 11 the calculus of risk changed," he said.

"If September 11 had not happened, our assessment of the risk of allowing Saddam any possibility of him reconstituting his programmes would not have been the same.

"After September 11, our view, the American view, changed and changed dramatically."

He said he wanted to make clear that countries developing weapons of mass destruction "had to stop".

"The primary consideration for me was to send an absolutely powerful, clear and unremitting message that, after September 11, if you were a regime engaged in WMD, you had to stop," he said.

"My assessment of risk prior to September 11 was that Saddam was a menace, that he was a threat, he was a monster, but we would have to try and make best."

Mr Blair insisted that he kept open his options for dealing with Iraq in the run-up to the Crawford talks.

An "options paper" was drawn up in March 2002 which outlined the choices open to Britain for tackling Saddam, including continuing the current containment policy with "smart sanctions", as well as regime change.

Mr Blair said he consulted foreign secretary Jack Straw, defence secretary Geoff Hoon and Ministry of Defence officials about the courses of action open to him.

He also held a meeting at his country residence, Chequers, before flying to Texas.

Mr Blair said: "We did have a very structured debate with the people. I mean, the fact that it happened at Chequers rather than Downing Street I don't think is particularly relevant to it.

"I think the simple answer is. did we consider those other options? - absolutely. That's why we had the paper drawn up."

Mr Blair sought to play down his comments in a BBC interview with Fern Britton in which he said he would have thought it right to remove Saddam, even if he had known that he did not have WMD.

"Even with all my experience in dealing with interviews, it still indicates that I have got something to learn about it," he said.

"I didn't use the words 'regime change' in that interview and I didn't mean in any sense to change the basis.

"Obviously, all I was saying was you cannot describe the nature of the threat in the same way if we knew then what we know now.

"It was in no sense a change of position. The position was that it was the approach of UN resolutions on WMD. That was the case. It was then and it remains."

Earlier, Mr Blair slipped past angry anti-war protesters outside the inquiry venue, the Queen Elizabeth II Conference Centre in Westminster, to enter by a cordoned-off rear entrance at about 7.30am.

He will give around six hours of evidence, watched in the small inquiry chamber by families of some of the 179 British personnel who died in the conflict.

Mr Blair will be asked when he committed Britain to overthrowing Saddam Hussein, whether he leaned on Attorney General Lord Goldsmith to agree the war was legal, and whether he manipulated intelligence about Iraq's supposed weapons of mass destruction.

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