A US military study officially acknowledged for the first time yesterday that Saddam Hussein had no direct ties to al-Qaida, undercutting the Bush administration's central case for war with Iraq.

The Pentagon study based on more than 600,000 documents recovered after US and UK troops toppled Hussein in 2003, discovered "no 'smoking gun' (ie, direct connection) between Saddam's Iraq and al-Qaida", its authors wrote. George Bush and his senior aides have made numerous attempts to link Saddam Hussein and Al Qaeda terror in their justification for waging war against Iraq.

As early as 2002, military intelligence analysts discounted the administration's claim that the Hussein government had trained al-Qaida members to employ chemical weapons. But Bush aides continued asserting that the intelligence they received showed a link.

"The reason I keep insisting that there was a relationship between Iraq and Saddam and al-Qaida: because there was a relationship between Iraq and al-Qaida," Bush said in 2004.

The Pentagon's independent auditor released a report last year that chastised an internal military office created by Bush allies for promoting a link between Hussein and al-Qaida despite intelligence showing that none existed. Even after that report, however, vice president Dick Cheney continued saying that al-Qaida leader Abu Musab al-Zarqawi collaborated with Iraq.

The Pentagon's treatment of the report today echoes its response in 2005 to a study prepared by another research arm that chastised the White House for failing to prepare for the post-war reconstruction of Iraq. Military officials deemed those conclusions "of a limited value" and decided to keep the study secret, the New York Times reported.