Mr David Kay stepped down as leader of the US hunt for banned weapons in Iraq last night and said he did not believe the country had any large stockpiles of chemical or biological weapons.
In a direct challenge to the Bush administration, which says its invasion of Iraq was justified by the presence of illicit arms, Mr Kay said in a telephone interview he had concluded there were no Iraqi stockpiles to be found.
"I don't think they existed," Mr Kay said. "What everyone was talking about is stockpiles produced after the end of the last (1991) Gulf War, and I don't think there was a large-scale production program in the '90s," he said.
The CIA announced earlier that former UN weapons inspector Mr Charles Duelfer, who has expressed his own doubts that unconventional weapons would be found, would succeed Mr Kay as Washington's chief arms hunter.
Mr Kay said he believes most of what was going to be uncovered in Iraq had been found and that the weapons hunt would become more difficult once America returned control of the country to the Iraqis.
The US went to war against Baghdad last year citing a threat from Iraq's weapons of mass destruction. So far, no banned arms have been found.
In his annual State of the Union address on Tuesday, US President George W. Bush insisted that former Iraqi leader Saddam Hussein had actively pursued dangerous weapons programs right up to the start of the U.S. attack in March.
"Had we failed to act," Mr Bush said, "the dictator's weapons of mass destruction programs would continue to this day."