Hans Blix, the former UN weapons inspector to Iraq said the US and UK relied on unreliable intelligence on the potential threat posed by Saddam Hussein ahead of the 2003 invasion.
"When we reported that we did not find any weapons of mass destruction they should have realised ... both in London and in Washington, that their sources [on weapons] were poor," he told a public inquiry in London on Tuesday.
"Their sources were looking for weapons, not necessarily weapons of mass destruction. They should have been more critical of that."
Blix said that his team of inspectors had visited 30 sites based on tip-offs from British and US intelligence but found little other than old missile engines and a sheaf of nuclear documents.
The Swedish diplomat, who has been an outspoken critic of the war, told five-member inquiry panel that it was "very hard" for Iraq to declare a weapons programme that did not exist.
He said UN resolution 1441, which required Saddam to co-operate with UN weapons inspectors, should have given Iraq the "chance for a new start".
"If they had weapons, which I thought might well be the case, they had an opportunity. Now here it is, they could put the blame on some general or
other," he said.
But when asked whether he felt the resolution, passed in November 2002, gave Iraq a realistic possibility of meeting the requirements, he said: "Yes, except that it was very hard for them to declare any weapons when they didn't have any".
Blix said he first became suspicious about the credibility of intelligence being used when the US and UK claimed Iraq was importing uranium from Niger.
The claims turned out to be based on forgeries that took the International Atomic Energy Agency just one day to discredit.
"I think that was the most scandalous part," he told the inquiry.
The former weapons inspector said the progress towards war in iraq was "almost unstoppable" by early 2003 and the UK was "a prisoner on that train".
Blix revealed earlier this year that he had urged Tony Blair in the month before the invasion, to consider the possibility that Saddam Hussein had no weapons of mass destruction.
He has also accused the UK and US governments of dramatising the limited intelligence on Iraq's weapons, saying: "The allied powers were on thin ice, but they preferred to replace question marks with exclamation marks".
Blix, who conducted inspections in Iraq from November 2002 to March 2003, had warned Saddam of "serious consequences" if he failed to co-operate with his team and comply with UN Security Council resolution 1441.
Earlier this year, Blair told the inquiry in London that Blix had been clear in his reports in the run-up to the war that Saddam was not complying with international demands.
"Hans Blix obviously takes a certain view now," he told the hearing.
"I have to say in my conversations with him then it was a little different."
Jack Straw, the former foreign secretary, also suggested that Blix may have applied a retrospective "gloss" to his actions at the time.
"There are some of those who were involved who sought to give an account of what they were saying at the time without gloss," he told the inquiry earlier this year.
"There are others who have sought to give an account of what they thought they were saying at the time with gloss, and I think the jury is out on which camp Dr Blix is in."
But critics of the war say that Blix should have been given more time to establish whether or not Saddam was hiding stocks of chemical, biological or nuclear weapons.
Blix is the first foreign witness to give evidence at a public hearing of the inquiry, though others have spoken to the five-member panel, headed by John Chilcot, a former civil servant, in private during visits to the US and France.