"Let's see what the chemicals are," Blix told a packed gathering of the Oxford Union debating club on Thursday, after Iraqi officials said they had uncovered a chemical bomb factory in Falluja.
"Many of these stories evaporate when they are looked at more closely," he told the mainly student crowd. "The chances [that the laboratory could produce weapons] are, I think, relatively small. I would be surprised if it was something real."
Blix, a former Swedish diplomat, searched for weapons of mass destruction in the 15 weeks leading up to the US-led invasion of Iraq in 2003.
In Baghdad, Iraqi security chief Qasim Dawud said national guardsmen had found a workshop in Falluja used to manufacture explosives and chemical substances.
US-led forces started the Falluja
offensive on 8 November
"In a house in the industrial district, in southwest Falluja, national guardsmen discovered a chemical materials laboratory that was used to make explosives and toxic substances," he said.
"There were also pamphlets showing ways to make explosives, toxic substances, including anthrax," he said.
On Wednesday, US Lieutenant-Colonel Dan Wilson said troops had been surprised by the number of weapons found, describing it as capable of sustaining a "free-for-all in the city of Falluja for months".