Tony Blair today told the Iraq Inquiry that taking Britain to war was “right – full stop”.

He said that the terrorist attacks on New York’s Twin Towers “dramatically” changed the threat posed by a “psychopathic” Saddam Hussein.

And he said: “I would not have done Iraq if I had not thought it was right. Full stop.”

In opening exchanges for his highly-anticipated evidence before the Chilcot inquiry he said September 11, 2001 had made him fear that Saddam could not be stopped from threatening other countries by existing United Nations sanctions.

Mr Blair said the 9/11 attacks on the United States completely transformed British policy towards Saddam and said he believed “beyond doubt” that Saddam had weapons of mass destruction (WMD).

He said: “I would fairly describe our policy up to September 11 as doing our best, hoping for the best but with a different calculus of risk assessment. “

He added: “The point about those acts in New York is that, had they been able to kill more people than the 3,000, they would have.”

He told the inquiry Iraq had shown “10 years of defiance “ and had to be brought “back into compliance “.

But he said the New York atrocities were key to the change in the “assessment” of the security risk.

“That completely changed our assessment of where the risks for security lay,” he said.

Mr Blair insisted that he kept his options for dealing with Iraq open before meeting George Bush at the US president’s ranch in Crawford, Texas, in April 2002 .

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. Sir Christopher Meyer, the former British ambassador to Washington, previously told the inquiry that it appeared an agreement had been “signed in blood “ by Mr Blair and Mr Bush during the ranch meeting.

But Mr Blair responded: “I don’t think Christopher Meyer was ever at the critical meeting. “

Mr Blair was asked if he felt in 2002 that the link between terrorism and Iraq’s supposed WMD was a potential threat to the UK.

He replied: “Yes, because for the reasons that I have given. If Saddam, freed from sanctions, was able to pursue WMD programmes, I was very sure that at some point we were going to be involved in the consequences of that.” He added: “This is a profoundly wicked, I would say almost psychopathic, man. We were obviously worried that after him his two sons seemed to be as bad, if not worse.”

Mr Blair said Saddam Hussein had used weapons on mass destruction “not only on his own people but thousands in the Iran-Iraq war”.

He said Downing Street had not added the claim that Saddam could launch WMD in 45 minutes to the intelligence dossier that set out the case for war.

He accepted it had been a mistake not to make clear that the now-notorious claim that some WMD could be launched within 45 minutes referred to battlefield weapons and not long-range missiles.

Mr Blair said: “It’s a decision. And the decision I had to take was, given Saddam’s history, given his use of chemical weapons, given the over one million people whose deaths he had caused, given 10 years of breaking UN resolutions, could we take the risk of this man reconstituting his weapons programmes or is that a risk that it would be irresponsible to take? “

He went on: “I had to take the decision. I believed, and in the end the Cabinet believed – so did Parliament incidentally – that we were right not to run that risk. “

Arriving at the Queen Elizabeth II Conference Centre at 7.30am, two hours early, to give evidence the former prime minister avoided protesters, entering the inquiry venue by a cordoned off rear entrance.

There was fury among the demonstrators outside the conference centre that the former premier had not faced them as he arrived.

The inquiry has already heard evidence suggesting that he agreed to join the US-led invasion nearly a year before it began.

Alastair Campbell, the former prime minister’s communications director, said Mr Blair sent US President George Bush secret messages in 2002, assuring him that Britain would “be there “ if it came to military action.

There were also questions about his interview with TV presenter Fern Britton in which Mr Blair claimed it would have been right to remove Saddam Hussein even without WMD.

More than 3,000 members of the public applied for 80 places to see Mr Blair give evidence live in the inquiry chamber – 40 in the morning and 40 in the afternoon.

Twenty seats in the room were set aside for the families of service personnel who died in the Iraq conflict.