By Julian Borger

President George Bush's attempt to maintain public support for military action against Iraq has taken a fresh blow from an unexpected quarter, with the publication of a letter from the CIA stating that while Saddam Hussein poses little threat to America now, a US invasion could push him into retaliating with chemical or biological weapons.

The unusually detailed public statement, in the form of a letter from the CIA director, George Tenet, to Congress, comes at a highly sensitive moment, potentially damaging Mr Bush's attempt to rally an overwhelming congressional mandate for the use of force against Iraq.

In a chilling excerpt, Mr Tenet warned that if Saddam was personally threatened he might seize "his last chance to exact vengeance by taking a large number of victims with him".

The risk of such an attack, possibly involving weapons of mass destruction, would rise from "low" to "pretty high" were Saddam to feel cornered by US military might.

Such a stark judgment seems likely to increase public anxiety about the prospect of a new war. There is still majority backing for military action, but that support appears to be fading despite a concerted public relations campaign by the administration to put its case. Approval for military action has fallen from 57% last month to 53% this week, according to a US Gallup poll.

The CIA letter was seized on by Democrat opponents of military action, at the height of the congressional debate on a resolution authorising an invasion if and when the president deems it necessary.

Donald Payne, a House Democrat, said that Mr Tenet's letter showed that the Bush administration's aggressive strategy "could trigger the very things that our president has said that he is trying to prevent: the use of chemical or biological weapons. In view of this report, the policy of a pre-emptive strike is troublesome."

Mr Tenet's letter came in response to a congressional request to declassify segments of CIA briefings on Iraq over the past few days. He said: "Baghdad for now appears to be drawing a line short of conducting terrorist attacks with conventional or CBW [chemical and biological weapons] against the United States."

This assessment is reinforced by testimony given to Congress last week by an unnamed senior intelligence officer, which Mr Tenet allowed to be declassified.

The officer said: "My judgment would be that the probability of [Saddam] initiating an attack . . . in the foreseeable future, given the conditions we understand now, the likelihood I think would be low." Asked about the likelihood of an Iraqi chemical or biological attack on the US in response to an invasion, the intelligence officer said: "Pretty high, in my view."

Mr Tenet emphasised the same point in his own words. "Should Saddam conclude that a US-led attack could no longer be deterred, he probably would become much less constrained in adopting terrorist actions," he wrote. He added that Saddam might work with Islamist terrorists to carry out an attack.

It is unusual for the CIA to put such details of its intelligence assessments into a public document. The letter was produced after intense pressure from senators. The letter also comes at a time when the CIA is competing with the more hawkish Pentagon, which is also supplying the White House with intelligence on the Iraqi threat.

"You have to ask yourself the question, since Tenet is part of the team, why now?" said Fred Hitz, a former CIA inspector general. "You have to go back to the Vietnam era to find a time when the judgment of the intelligence community was in the public eye on such a current affairs basis." The White House last night denied that the CIA analysis undermined Mr Bush's message on the urgency of confronting Baghdad.

Mr Tenet "did not say we're OK," the White House spokesman, Ari Fleischer, said. "If Saddam Hussein holds a gun to someone's head, while he denies he even owns a gun, do you really want to take a chance that he'll never use it."

In a bid to dampen the controversy, Mr Tenet later put out a statement insisting: "There is no inconsistency between our view of Saddam's growing threat and the view as expressed in [Bush's] speech.

"Although we think the chances of Saddam initiating a WMD [weapons of mass destruction] attack at this moment are low, in part because it would constitute an admission that he possesses WMD, there is no question that the likelihood of Saddam using WMD against the US or our allies in the region for blackmail, deterrence or otherwise grows as his arsenal continues to build."

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