In a direct challenge to the Bush administration, which says its invasion of Iraq was justified by the presence of illicit arms, Kay told Reuters in a telephone interview he had concluded there were no Iraqi stockpiles to be found.

"I don't think they existed," 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 named former UN weapons inspector Charles Duelfer, who had expressed his own doubts that unconventional arms would be found in Iraq, to replace Kay.

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 in June.

Top Democrats on the congressional intelligence committees seized on Kay's comments.

"It increasingly appears that our intelligence was wrong about Iraq's weapons, and the administration compounded that mistake by exaggerating the nuclear threat and Iraq's ties to al-Qaeda. As a result, the United States is paying a very heavy price," said Senator John Rockefeller of West Virginia, the senior Democrat on the Senate Intelligence Committee.

"Dr Kay's astonishing statement today cannot be ignored. It is increasingly clear that there has been a massive intelligence failure," said Republican Jane Harman of California, senior Democrat on the House of Representatives Intelligence Committee.

James Rubin, national security adviser for retired General Wesley Clark, a candidate for the Democratic presidential nomination, said Kay's comments meant "the major premise for urgent war in Iraq has been devastated by the administration's own findings.

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No banned arms have been found in Iraq since the United States went to war against Baghdad last year.

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 programmes right up to the start of the US attack in March.

"Had we failed to act," Bush said, "the dictator's weapons of mass destruction programmes would continue to this day."

And on Wednesday, Vice President Dick Cheney said the United States had not given up on finding banned weapons in Iraq. "The jury is still out," he said in a radio interview.

White House spokesman Scott McClellan said on Friday, in response to Kay's remarks, "We remain confident that the Iraq Survey Group will uncover the truth about Saddam Hussein's regime, the regime's weapons of destruction programmes."

Kay said he left his post due to a "complex set of issues," including a reduction in resources and a change in focus of the Defence Department's Iraq Survey Group, which is conducting the weapons hunt, toward fighting against the insurgency.