The all-party report by the 9/11 Commission, set up by Congress in 2002, will state that Iran, not Iraq, fostered relations with the al-Qa'eda network in the years leading up to the world's most devastating terrorist attack.

The bipartisan commission has established that between eight and 10 of the September 11 hijackers, who had been based in Afghanistan, travelled through Iran between October 2000 and February 2001. The terrorists in question are believed to have been the "muscle" - hired to storm the aircraft cockpits and overpower crew and passengers.

Iranian officials were instructed not to harrass al-Qa'eda personnel as they crossed the border and, in some cases, not to stamp their passports.

According to testimony received by the commission - based on information from prisoners at Guantanamo Bay and about 100 electronic intercepts by the National Security Agency - an alliance of convenience was established between the Shia Muslim Iranian leadership and the Sunni terrorist organisation, well before September 11, 2001.

The report is expected to confirm the claim by Thomas Kean, its chairman, last month that "there were a lot more active [al-Qa'eda] contacts, frankly, with Iran and Pakistan, than there were with Iraq".

It will further inflame tensions between Washington and Teheran, where hardliners are threatening to restart its uranium enrichment programme, a key step towards building nuclear weapons.

A commission official, quoted in the latest edition of Time magazine, alleges that Iranian officials approached Osama bin Laden after the bombing of the USS Cole in 1999, proposing a joint strategy of attacks on US interests.

A preliminary report from commission staff, released last month, stated: "Bin Laden's representatives and Iranian officials discussed putting aside Shia-Sunni divisions to co-operate against the common enemy."

The offer is said to have been turned down by bin Laden, who was reluctant to alienate Sunni supporters in Saudi Arabia. Nevertheless, in the wake of September 11, Iran sheltered al-Qaeda militants fleeing Afghanistan.

The full report by the commission is also expected to endorse initial conclusions that al-Qa'eda may have been involved in the 1996 bombing of the Khobar Towers complex in Saudi Arabia, when 19 American servicemen were killed. The attack has long been blamed solely on Hizbollah, a Lebanese terrorist group backed by Iran.

Iran was declared part of an "axis of evil", along with Iraq and North Korea, by President George W. Bush in 2002. The report will add to pressure for Iran's theocratic rulers to be the first target of a re-elected Bush administration. Hawks within the administration want a concerted effort to overturn the regime by peaceful means.

Some Bush officials are privately contemplating a possible military strike against Iran's nuclear facilities before Russian fuel rods are delivered next year.

Teheran said yesterday that it had arrested an unspecified number of Iranian al-Qa'eda supporters.