A US Senate Intelligence Committee report this week will sharply criticise the CIA for being predisposed to believing Iraq had weapons of mass destruction before the US-led attack that deposed Saddam Hussein.
The report, scheduled for public release tomorrow, will say that intelligence analysts did not question the long-held belief that Iraq had banned weapons and saw ambiguous information as supporting that view, a Senate source said.
The report is also expected to criticise intelligence agencies for using unreliable and inadequate sources.
"They used the thinnest sources to justify the grandest conclusions about weapons of mass destruction and other activity in Iraq," Senator Richard Durbin, an Illinois Democrat on the panel, said.
The main US justification for attacking Iraq was the claim that Saddam possessed stockpiles of chemical and biological weapons and was attempting to develop nuclear weapons. None, however, have been found and the US's main ally in the offensive, Britain has admitted there are almost certainly no such weapons in the country.
Republicans said the report is intended to be a constructive contribution to the debate on how to reform the US intelligence apparatus.
Some Democrats have written "additional views" to the report which will raise questions about whether the President George W Bush's administration, including the White House and Pentagon officials, pressured the CIA to tailor its conclusions to suit the administration's desire to gain control of Iraq.