The CIA declassified a letter Thursday that suggests US intelligence had grave doubts about part of the case made by former president George W. Bush's White House to justify the war in Iraq.

In the run-up to the March 2003 invasion, US officials including then vice president Dick Cheney alleged that 9/11 hijacker Mohammed Atta had met an Iraqi spy in Prague before the attacks.

The alleged meeting was cited as evidence of a possible link between Iraqi dictator Saddam Hussein and the September 11, 2001 attacks on New York and Washington.

But, in a letter sent to US Senator Carl Levin in March this year and just now declassified, CIA Director John Brennan said field agents had "expressed significant concern" over the report.

The letter said US agents had not established Atta was in Prague -- evidence suggest that he was not -- at the time he is supposed to have met Iraqi agent Ahmad Khalil Ibrahim Samir al-Ani.

The letter was first reported by McClatchy newspapers.

Levin said on Thursday he had asked the CIA to declassify the document to show how the former Bush administration "misled" the country before the invasion of Iraq.

In a Senate speech, Levin said the "alleged meeting was a centerpiece of the administration's campaign to create an impression in the public mind that Saddam was in league with the Al-Qaeda terrorists who attacked us on 9/11."

"On multiple occasions, including national television appearances, Vice President Dick Cheney cited reports of the meeting, at one point calling it 'pretty well confirmed,'" Levin said.

Levin said he raised the issue to give "the American people a full account of the march to war as new information becomes available."

Levin added that the revelation "is about warning future leaders of this nation that they must not commit our sons and daughters to battle on the basis of false statements."