Hans Blix, the former head of UN weapons inspectors to Iraq, has begun giving his testimony to a British public inquiry into the 2003 conflict.
The Swedish diplomat, who previously called the invasion a "tragedy" and "spectacular failure", is expected to speak about his tense relationships with former US and UK leaders in the run up to the war.
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.