Hans Blix said the United States and Britain based their weapons assessments on poor quality information.
"They should have realized, I think, both in London and in Washington that their sources were poor," Blix said. "Their sources were looking for weapons, not necessarily weapons of mass destruction. They should have been more critical of that."
Blix said he warned then British Prime Minister Tony Blair in a February 2003 meeting — and in separate talks, US Secretary of State Condoleezza Rice — that Saddam Hussein might have no weapons of mass destruction.
He said he told Rice and Blair his "belief, faith in intelligence had been weakened."
Though Blix had made such comments before, his testimony at the inquiry built on that of others about the dubious intelligence that existed before the war and increasingly drew a picture of the United States inevitably marching to conflict and ignoring any evidence to the contrary.
Blix said he believed Blair — who has also testified before the inquiry — was genuine in insisting the Iraqi leader was hiding weapons and defying the international community.
"I certainly felt that he was absolutely sincere in his belief," Blix said. "What I questioned was the good judgment."
The five-member Iraq inquiry panel was set up by the British government to examine the case made for the war, including intelligence, and errors in planning for post-conflict reconstruction.
Blix has repeatedly claimed in the past that inspectors had too little time to assess whether or not Saddam was concealing weapons of mass destruction, as the United States and Britain believed.
He said that, immediately before the 2003 US-led invasion, his inspectors checked around three dozen sites said by British and US intelligence to contain such weapons, but discovered no evidence.
"I made the remarks, which I've cited many times, that wouldn't it be paradoxical for you to invade Iraq with 250,000 men and find very little," Blix told the inquiry, referring to the Blair meeting.
The 82-year-old ex-inspector said he believed the US was "high" on military action in the aftermath of the Sept. 11, 2001 attacks, and dismissive of opposing views.
Blix told the British panel that the US was untroubled by issues over the authorization from the UN Security Council of military action.
"The US in 2002 threw it overboard, I think they were high on military at the time. They said: 'We can do it,'" Blix said.
"I think there was at least implied from the US that if the Security Council doesn't agree with us and go along with our view, then it sentences itself to irrelevance. That is, I think, a very presumptuous attitude," he told the panel.
Blix referred to suggestions that US and British intelligence tapped phones and bugged offices at the UN in the run-up to war. "Some people thought we were bugged in New York," he said. "My only complaint about that is they could have listened more carefully to what we had to say."
The inquiry won't apportion blame or establish criminal or civil liability and has heard testimony from politicians and military and intelligence officials, including Blair.
Blix's testimony follows concerns raised last week by Eliza Manningham-Buller, ex-director of Britain's domestic spy agency MI5, that the prewar intelligence picture was "fragmentary."
"The picture was not complete. The picture on intelligence never is," she told the panel.
