The September 11 attacks changed the "calculus of risk" and meant it was no longer possible to contain Saddam Hussein through sanctions, former British prime minister Tony Blair said on Friday, explaining why he backed the 2003 US-led invasion that toppled the Iraqi dictator. Blair was appearing before an inquiry into Britain's role in the war, to which he committed 45 000 troops. It was the most controversial episode of his 10-year premiership, provoking huge protests, divisions within his Labour Party and accusations he had deceived the public about the justification for invasion. Under close questioning, Blair said the September 11 2001 al-Qaeda attacks on the United States, and the threat of weapons of mass destruction (WMD), meant that the policy of containing Saddam could not continue. "Up to September 11, we thought he was a risk but we thought it was worth trying to contain it. The crucial thing after September 11 is that the calculus of risk changed," Blair said. "The point about this act in New York was that had they been able to kill even more people than those 3 000, they would have. And so after that time, my view was you could not take risks with this issue at all. "We were advised, obviously, that these people would use chemical or biological weapons or a nuclear device if they could get hold of them, that completely changed our assessment of where the risks for security lay." Seven years after the invasion that toppled Hussein, and almost three years after Blair handed over to Gordon Brown, the issue still provokes deep public anger. 'War criminnal' Blair's appearance has been hugely anticipated. Protesters chanting "Tony Blair, war criminal" gathered outside the building opposite Parliament where the inquiry was taking place. Relatives of some of the 179 British soldiers killed in Iraq joined about 100 anti-war demonstrators chanting and waving placards. Names of those killed were also read out.