"It is smoke and mirrors. It has nothing to do with reality. It was plain wrong," Ritter said in a speech to ministers, diplomats and journalists in the United Arab Emirates.

Powell used audiotapes of intercepted Iraqi conversations and satellite photos to argue Iraq concealed arms by bulldozing chemical weapons sites, hiding rocket launchers under palm trees and moving mobile biological weapons labs on trucks.

But Ritter said he knew from his experience as a weapons inspector that none of the evidence stood up to scrutiny.

He said satellite images were of limited use without further investigation, as suspicious images often turned out to have innocuous explanations, and radio intercepts were worthless unless the context of the conversation was known.

The testimony of defectors was also of limited use, Ritter said, especially as they gave conflicting evidence on whether Iraq had retained weapons of mass destruction.

Ritter resigned as a U.N. weapons inspector in 1998, saying inspectors had insufficient backing to do their jobs. At the time he said Iraq had not disarmed and still posed a threat.

His critics say his switch to the view that Iraq has no banned weapons lacks credibility, and question his motives. But Ritter says his arguments are based on hard evidence.

MOBILE BIOWEAPONS LABS

Ritter said there was no evidence to support Powell's assertion that Iraq has at least 18 mobile bioweapons labs.

The possibility that Iraq had such mobile laboratories had been raised by inspectors in the 1990s purely as a hypothetical way in which Iraq could be concealing weapons, Ritter said.

"These labs exist purely in the minds of inspectors," he said. "We hypothesised their existence. There is no information to say they ever existed. We made them up. But they have taken on a life of their own."

Ritter said it was true Iraq had not accounted for some ingredients used in the production of anthrax, but the last known batch of liquid bulk anthrax had been produced in 1991, at a factory destroyed in 1996.

Even under ideal storage conditions, he said, within three years liquid bulk anthrax becomes "useless sludge".

Ritter said recent Iraqi concessions, including allowing some scientists to be interviewed without minders present, meant a credible inspections process could get under way in Iraq.

But he added that Washington's main aim was to depose Iraqi President Saddam Hussein, not Iraqi disarmament, so the United States would block attempts to set up long-term inspections.

"Let there be no doubt that Iraq did possess weapons of mass destruction," he said. "But Iraq no longer possesses a meaningful capacity to produce weapons of mass destruction."