"I'm certainly more and more to the conclusion that Iraq has, as they maintained, destroyed all almost of what they had in the summer of 1991," Blix told Australian national radio.
Blix was chief UN weapons inspector on Iraq in the run-up to the war. His inspectors worked in Iraq for several months in late 2002 and early 2003 and failed to find conclusive evidence of the alleged weapons.
Their absence has become a major embarrassment for the United States and its allies, who used it to justify their invasion in March. The United Nations refused to endorse the war.
Asked if it was likely Iraq has not had weapons of mass destruction for at least 10 years, Blix said: "Yup, that's right."
But the former Swedish diplomat said there had to be an explanation why the Iraqi authorities had created so many difficulties for his inspectors, suggesting it might have been a complex charade, designed precisely to prevent an invasion.
"You see, if they didn't have anything (WMD) after '91, there must be some explanation why they behaved as they did. They certainly gave the impression that they were denying access and so forth," Blix said.
"I mean, you can put up a sign on your door, Beware of the Dog, without having a dog."
Blix's suggestions, which he said were already known in the United States, called into question controversial intelligence used by Britain and the United States to justify the conflict in Iraq.
On December 7 last year, the government of then-Iraqi president Saddam Hussein made a submission to the United Nations in which it said it did not have chemical, biological or nuclear weapons.
It was quickly dismissed as false and incomplete by the United States and Britain, which accused Baghdad of failing to disarm as required by Security Council Resolution 1441. These charges were later used by Washington and London to justify the invasion of the country in late March.
Since then, British Prime Minister Tony Blair has been under fire for allegedly ignoring intelligence casting doubt on the case for war.
And Australian Prime Minister John Howard this week was accused of lying over a British intelligence report suggesting a strike on Baghdad would increase the likelihood of terrorist attacks against Western targets.
US officials have also been increasingly keen to downplay the significance of the search for weapons in Iraq. The US-controlled Iraq Survey Group has been scouring the country for evidence of weapons, but its hundreds of scientists have found very little.
Blix pointed out that the rhetoric of official descriptions of the hunt for weapons had been progressively weakened, doubting that any would now be found.
"The more time that has passed, the more I think it's unlikely that anything will be found," Blix said.
"In the beginning they talked about weapons concretely, and later on they talked about weapons programs, and maybe they'll find some documents of interest but that should have surfaced and, I think, explained."
Saddam's regime was repeatedly accused of using chemical weapons against his own people and during the long-running war against Iran in the 1980s. But after the 1991 Gulf war his regime was isolated and starved of funds under UN sanctions.
The United States failed to bring down Saddam in 1991, despite calling on Shiite Muslims in the south of the country to rise up against him, a call that resulted in a heavy backlash against them by government forces.
Since its successful invasion, the US-led coalition has become increasingly bogged down in Iraq, facing strident calls to leave the country, a wave of attacks against its forces and a series of bombings, some blamed on al-Qaeda.