There is no evidence of formal links between Iraqi ex-leader Saddam Hussein and al-Qaeda leaders prior to the 2003 war, a US Senate report says.
Democrats say the report weakens Mr Bush's case for war
The finding is contained in a 2005 CIA report released by the Senate's Intelligence Committee on Friday.
US President George W Bush has said that the presence of late al-Qaeda leader Abu Musab al-Zarqawi in Iraq before the war was evidence of a link.
Opposition Democrats are accusing the White House of deliberate deception.
They say the revelation undermines the basis on which the US went to war in Iraq.
The BBC's Justin Webb in Washington says that the US president has again and again tried to connect the war, which most Americans think was a mistake, with the so-called war on terror, which has the support of the nation.
The report comes as Mr Bush makes a series of speeches on the "war on terror" to coincide with the fifth anniversary of the 11 September attacks.
The report is the second part of the committee's analysis of pre-war intelligence. The first dealt with CIA failings in its assessment of Iraq's weapons programme.
The committee concluded that the CIA had evidence of several instances of contacts between the Iraqi authorities and al-Qaeda throughout the 1990s but that these did not add up to a formal relationship.
It added that the government "did not have a relationship, harbour or turn a blind eye toward Zarqawi and his associates".
It said that Iraq and al-Qaeda were ideologically poles apart.
"Saddam Hussein was distrustful of al-Qaeda and viewed Islamic extremists as a threat to his regime, refusing all requests from al-Qaeda to provide material or operational support," it said.
The Senate report added that the Iraqi regime had repeatedly rejected al-Qaeda requests for meetings.
It also deals with the role played by inaccurate information supplied by Iraqi opposition groups in the run-up to the war.
Democrats said the White House was still trying to make the connection between the former Iraqi leader and al-Qaeda in an attempt to justify the war in Iraq.
Less than three weeks ago Mr Bush said in a speech that "Saddam Hussein...had relations with Zarqawi".
Democrat Senator Carl Levin described the report as a "devastating indictment" of these attempts.
White House spokesman Tony Snow told the Associated Press news agency the report contained "nothing new".
"In 2002 and 2003, members of both parties got a good look at the intelligence we had and they came to the very same conclusions about what was going on," he said.
Zarqawi, who is believed to be responsible for numerous killings and kidnappings in Iraq since the war, was killed in a US raid in June.
Saddam Hussein and several close associates are standing trial for the killings of Shias in the village of Dujail in the early 1980s and of more than 100,000 Kurds in 1988.