After four months of poring over 19 volumes of classified material used by the White House to justify its case for war, senior members of the committee concluded that there were "significant deficiencies" in the community's ability to collect fresh intelligence on Iraq. They said it had to rely on past assessments, dating to when UN inspectors left Iraq in 1998, and on "some new 'piecemeal' intelligence", both of which "were not challenged as a routine matter".

In a letter to the CIA director, George Tenet, that was leaked to the Washington Post, two committee members claimed: "The absence of proof that chemical and biological weapons and their related development programs had been destroyed was considered proof that they continued to exist. The assessment that Iraq continued to pursue chemical and biological weapons remained constant and static over the past 10 years."

The letter is all the more damaging because it comes from a committee controlled by Republicans and is signed by the committee chairman, Congressman Porter Goss, a Republican from Florida who is a former CIA agent and a long-time supporter of Mr Tenet and the intelligence agencies.

Their findings echoed claims made by the United Nations chief weapons inspector, Hans Blix, two weeks ago that most of Iraq's weapons of mass destruction were destroyed 10 years ago.

"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," said Mr Blix. "The more time that has passed, the more I think it's unlikely that anything will be found."

The committee's conclusions also have striking parallels in much of the evidence that has emerged from the Hutton inquiry in London, that the intelligence agencies came up with evidence to support the political demands the government to go to war.

Regarding Iraq's alleged ties to al-Qaida, the letter argues that the agencies had a "low threshold" or "no threshold" on using the information they gathered.

"As a result, intelligence reports that might have been screened out by a more rigorous vetting process made their way to the analysts' desks, providing ample room for vagary to intrude," the letter states. "The agencies did not clarify which of their reports were from sources that were credible and which were from sources that would otherwise be dismissed in the absence of any other corroborating intelligence."

"To attempt to make such a determination so quickly and without all the facts is premature and wrong," Bill Harlow, an agency spokesman, told the Washington Post. "Iraq was an intractable and difficult subject. The tradecraft of intelligence rarely has the luxury of having black-and-white facts. The judgments reached, and the tradecraft used, were honest and professional - based on many years of effort and experience."

The national security adviser, Condoleezza Rice, yesterday also disputed the claims. She told Fox News Sunday: "There was enrichment of the intelligence from 1998 over the period leading up to the war. And nothing pointed to a reversal of Saddam Hussein's very active efforts to acquire weapons of mass destruction ... it was very clear that this continued, and it was a gathering danger."

News of the criticism comes at a difficult time for President George Bush, who came away from a week of trying to persuade foreign leaders to give financial and military assistance to maintain security in Iraq empty handed and with his approval ratings plummeting.

The first lady, Laura Bush, set out to soften America's image abroad this week with a European tour to France and Russia. "There's a great benefit for our country if we can really let people around the world know what we are really like and what our values are really like," she said yesterday.