The head of the United Nations inspection team which hunted for Saddam Hussein's supposed weapons of mass destruction in the months before the 2003 invasion of Iraq will today give evidence to the Chilcot Inquiry into the war.
Hans Blix's appearance is likely to heighten pressure on Tony Blair, as he tells the Inquiry about his increasingly tense contacts with the former British Prime Minister and then US President George Bush as war drew nearer.
Critics of the war often claimed that Dr Blix should have been given more time to establish whether or not Saddam was hiding stocks of chemical, biological or nuclear weapons material.
But Mr Blair himself told the Inquiry in January that he did not believe the result would have been different even if Dr Blix's Unmovic team was given a further six months.
The Swedish diplomat - who described the invasion as a "tragedy" and a "spectacular failure" - revealed earlier this year that he urged Mr Blair a month before the invasion to consider the possibility that Saddam had no WMD.
Dr Blix, who led Unmovic from 2000-03 and conducted inspections in Iraq from November 2002 to March 2003, warned Saddam of "serious consequences" if he failed to co-operate with his team and comply with UN Security Council resolution 1441.
But after the war, he accused the US and UK governments of dramatising the limited intelligence on WMD, saying: "The allied powers were on thin ice, but they preferred to replace question marks with exclamation marks."
Last weekend, Dr Blix said: "I question their good judgment. I never gave up hope for a non-military development until we were told to move our inspectors out. That was a few days before the military action started."
In his own evidence to the Inquiry in January, Mr Blair insisted that Dr Blix was clear in all his reports in the run-up to war that Saddam was not complying with international demands.
"Hans Blix obviously takes a certain view now," the former PM told the Inquiry. "I have to say in my conversations with him then it was a little different."
Former British Foreign Secretary Jack Straw suggested that Dr Blix may have applied a retrospective "gloss" to his actions at the time.
Mr Straw told the Inquiry: "There are some of those who were involved who sought to give an account of what they were saying at the time without gloss.
"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."
During his appearance before Chilcot in January, Mr Straw was also critical of the weapons inspector for failing to table a key document at a meeting of the UN Security Council just days before the invasion - and even suggesting he had not read it.
Dr Blix is the first foreign witness to give evidence at a public hearing of the Inquiry, though others have spoken to John Chilcot's panel in private during visits to the US and France.