"When Clinton was here recently he told me he was absolutely convinced, given his years in the White House and the access to privileged information which he had, that Iraq possessed weapons of mass destruction until the end of the Saddam regime," he said in an interview with Portuguese cable news channel SIC Noticias.
Clinton, a Democrat who left office in 2001, met with Durao Barroso on October 21 when he travelled to Lisbon to give a speech on globalization.
The US justified going to war against Iraq last year citing the threat posed by Baghdad's weapons of mass destruction.
Republican President George W. Bush used Iraq's nuclear, chemical and biological weapons programmes and Saddam Hussein's ties to terrorism as the main case to the United Nations for the US-led war against Iraq.
But since the US occupation of Iraq, American forces have failed to uncover any chemical, biological or nuclear weapons since the war. Hundreds of experts are still scouring Iraq in the hunt.
An influential Washington think-tank said Thursday the Bush administration "systematically" inflated the threat from Iraq's weapons programmes in a bid to strengthen its push for military action against Iraq last year.
In its report, the Carnegie Endowment for International Peace also said it was unlikely that Iraq could have destroyed, hidden or moved out of the country hundreds of weapons of mass destruction without Washington detecting some sign of activity.