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.

Wary of embarrassing press coverage noting that the new study debunks those claims, the US defence department attempted to bury the release of the report yesterday.

The Pentagon cancelled a planned briefing on the study and scrapped plans to post its findings on the internet, ABC news reported. Unclassified copies of the study would be sent to interested individuals in the mail, military officials told the network.

Another Pentagon official told ABC that initial press reports on the study made it "too politically sensitive".

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.

Government memos examined by US military researchers showed financial support given to "suicide bombers in Gaza and the West Bank", the study continued. The Hussein regime also recorded its development of car bombs and explosive vests in detail.

Despite those findings, the admission of no direct ties between the deceased dictator and al-Qaida makes the study a potential flashpoint for the Bush administration as it tries to play up signs of improving security in Iraq. Next week marks the fifth anniversary of the invasion, and several high-profile remembrance events are planned in Washington.

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.

Al-Zarqawi "took up residence there before we ever launched into Iraq, organized the al-Qaida operations inside Iraq before we even arrived on the scene," Cheney told conservative radio host Rush Limbaugh in April.

The documents used for today's study came from the so-called Harmony database, an expansive catalogue of al-Qaida evidence maintained by the military.

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.

Authors of the study-conducted by the Institute for Defence Analyses, the military's non-profit research arm, found evidence that the Hussein government supported some Palestinian terrorist groups but largely focused their attention on internal terrorist acts.

"The predominant targets of Iraqi state terror operations were Iraqi citizens, both inside and outside of Iraq," the study's executive summary concluded.