Former British prime minister Tony Blair has told the Iraq War Inquiry the September 11 attacks changed the "calculus of risk" and meant it was no longer possible to contain Saddam Hussein through sanctions.

Mr Blair was appearing before the inquiry to explain why he backed the 2003 US-led invasion, 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, Mr 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," Mr 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 Saddam Hussein and almost three years after Mr Blair handed over to current British Prime Minister Gordon Brown, the issue still provokes deep public anger.

'War criminal'

Mr 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.

Mr Blair, who looked nervous at times as the hearing began, arrived early and entered by a back door amid heavy security and large numbers of police on stand-by.

"The real question Tony Blair needs to answer in the end will be at The Hague and before a war crimes tribunal," said Andrew Murray, chairman of Stop the War Coalition.

"He is an accomplished actor but I think most people have long since seen through the script."

Mr Blair's appearance will not only affect his own personal legacy but still has the potential to damage the Labour government of his successor Mr Brown, who was finance minister during the war.

Some Labour leaders fear it will reignite strong feelings on the issue among voters, denting support for a party already trailing the Conservatives in polls in the run-up to an election due by June.

The inquiry has already heard from senior civil servants who said intelligence in the days before the March 20, 2003 invasion indicated that Saddam's WMD had been dismantled.

It will also examine the war's legitimacy and at what stage Mr Blair, now an international envoy to the Middle East, promised then US president George W Bush that Britain would support military action against Iraq.

Witnesses have suggested Mr Blair gave that assurance in 2002 although then-attorney general Peter Goldsmith, the government's top lawyer who eventually gave the invasion the green light, had warned him that using force for regime change would be illegal.

Mr Goldsmith told the inquiry he originally believed the United Nations had to approve the use of force and only changed his mind a month before the invasion.

The two top lawyers at the time at the Foreign Office have also said they had told the government the war would be unlawful.

-Reuters