In six hours of scheduled hearings, many of the questions were likely to turn on his relationship with George W Bush after the attacks of September 11, 2001, particularly confidential meetings he had with Bush in 2002 to discuss the fervour for an enforced change of government in Iraq, even though there was no evidence that the Iraqi regime had anything to do with the attacks in New York and Washington.

Before September 11, Blair said on Friday, referring to Saddam Hussein, “We thought he was a risk, but it was worth trying to contain it. The crucial thing after September 11 is that the calculus of risk changed.”

“The point about this terrorist act was that over 3,000 people had been killed on the streets of New York and this is what changed my perception of risk: if these people inspired by this religious fanaticism could have killed 30,000, they would have.”

Discussing America’s plans after the September 11 attacks, Blair said: “I didn’t want America to feel it had no option but to do it alone.”

At a meeting with Bush in April 2002, at his ranch in Crawford, Texas, Blair testified, “What I said to George Bush was that we are going to be with you” in countering the perceived threat from Saddam Hussein.

Countering widespread speculation in Britain that the two men resolved to go to war at that meeting, he said there was no firm agreement on what specific action — diplomatic, military or economic — would be taken. Blair said he told Bush that “if it came to military action, we would be with” the US. But, at that time, Britain favoured diplomacy through the UN.

Blair acknowledged that the nature of the Iraqi regime sharpened his perception of Saddam Hussein as a threat not just to the region but to Britain. Calling Hussein a “probably wicked if not psychopathic man,” Blair said if he had been able to pursue a programme to develop weapons of mass destruction “at some point we were going to be involved in the consequences of that.”

He was also asked about a contentious British intelligence dossier published in September 2002 arguing that Iraq had unconventional weapons that could be used within 45 minutes of an order being given. Blair said that he believed “beyond doubt” that the Baghdad regime possessed unconventional weapons and the threat from them was growing.

As he arrived at Queen Elizabeth II conference centre, close to the Houses of Parliament on the banks of the River Thames, several hundred demonstrators gathered to vent enduring anger over the invasion.

Demonstrators, outnumbered by the police, chanted slogans like “Jail Tony” and “Blair Lied — Thousands Died.”

The inquiry, headed by Sir John Chilcot, a 70-year-old retired civil servant, follows two others that have sought to unravel the government’s handling of the invasion and the role played by information from intelligence agencies used by Blair to bolster his insistence that — “beyond doubt,” as he put it at the time — Saddam Hussein’s arsenal included weapons of mass destruction.