Feith presented the White House with claims of a "mature symbiotic relationship" between Iraq and al-Qaeda as if they were facts, while ignoring contradictory views from the intelligence community, Gimble said.
"They did not show the other, dissenting side," he said.
Paul Wolfowitz, the former deputy defence secretary, authorised Feith to pursue alternative intelligence conclusions telling him the action was lawful, Gimble concluded.
A claim by Feith's office that Mohamed Atta, one of the September 11 hijackers, met with an Iraqi official months before the 2001 attacks could not be verified by intelligence, he said.
Feith's actions were sometimes "inappropriate" because they "did not clearly show the variance with the consensus of the intelligence community," the report said.
As a result, Feith's office "did not provide 'the most accurate analysis of intelligence' to senior decision-makers," it said.
"The inspector general's report is a devastating condemnation," said senator Carl Levin, the senate panel's Democratic chairman.
Ike Skelton, Democratic chairman of the House of Representatives armed services committee, said the report showed that Feith's office exercised "extremely poor judgment for which our nation and our service members in particular, are paying a terrible price".
The report, requested by the senate intelligence committee in September 2005, recommended no action be taken because leadership changes in the Pentagon and intelligence community made a recurrence unlikely.
|The Pentagon's former policy chief, is said to|
have undermined CIA intelligence [AFP]
Feith, who left government in 2005, welcomed the finding that his activities were legal and authorised but said it was "absurd" to conclude that his work was inappropriate.
"It, of course, varied from [the] consensus. It was a criticism of that consensus. That is why it was written," he said in a statement.
Republicans loyal to George Bush, the US president, rebutted Levin and called Feith's work an intelligence critique that required no formal vetting process.
"I don't think in any way that his report can be interpreted as a devastating condemnation," said Republican senator James Inhofe.