Joseph Wilson, Washington's envoy to Gabon from 1992 to 1995, said in an article in the New York Times that he went to Niger in February 2002 at the request of the CIA.
His task was to assess the intelligence report - which the International Atomic Energy Agency later dismissed as being based on forged documents.
Before the IAEA gave its verdict, the report was cited by President George Bush and echoed by UK Prime Minister Tony Blair to support their charges that Saddam was trying to obtain nuclear weapons.
The false charge went some way to help justify the US-UK invasion and occupation of Iraq in March.
"Based on my experience with the administration in the months leading up to the war, I have little choice but to conclude that some of the intelligence related to Iraq's nuclear weapons program was twisted to exaggerate the Iraqi threat," Wilson wrote.
Controversy is raging in both Britain and the United States over charges that the governments of the two countries manipulated intelligence about weapons of mass destruction to justify the war.
|If there were no WMD, reasons|
for continued US occupation look
No evidence of such weapons has been found by the occupying forces in Iraq.
Wilson said he spent eight days in Niger meeting current and former government officials and people associated with the uranium business to check if there had been an Iraq-Niger deal.
"It did not take long to conclude that it was highly doubtful that any such transaction had ever taken place," he said.
Informed US administration
Wilson, who helped to direct Africa policy for the National Security Council under former President Bill Clinton, said the CIA would have passed on his findings to the office of Vice
President Dick Cheney.
Wilson noted that in January 2003 Bush "repeated the charges about Iraqi efforts to buy uranium from Africa".
"If the president had been referring to Niger, then his conclusion was not borne out by the facts as I understood them," he said.
Wilson said that if the administration had ignored his information "because it did not fit certain preconceptions about Iraq, then a legitimate argument can be made that we went to war under false pretenses."
According to news reports, the allegations of an Iraq-Niger deal were based on forged letters obtained by Italian intelligence from an African diplomat. The allegations were apparently passed to British intelligence and then to the CIA.