The inquiry has been given evidence that army members supplied loyalist terrorists with explosives that had been confiscated by the army from the IRA and that these were used in the three bombs which exploded in Dublin during rush-hour traffic.

The findings of the inquiry into the bombings, on May 17, 1974, are being written up by Mr Justice Henry Barron, a retired Irish judge. He has been forced to delay his inquiry repeatedly because British authorities have been slow to give him vital information. The report, due to be completed in April, will fuel persistent allegations that the bombings were engineered by undercover agents working inside loyalist paramilitary groups. A British Army bomb disposal expert has concluded that the three car bombs in Dublin used crystallised ammonium nitrate. The home bomb making technology was known to the IRA at the time but was not mastered by loyalists for at least another six years.

Originally, both loyalist and republican terrorists made most of their bombs from ammonium nitrate fertiliser mixed with fuel oil, but in 1973 restrictions introduced on the nitrate content of fertiliser made this simple process impossible.

IRA members quickly developed a crystallisation technique that allowed them to extract nitrates from the new fertiliser, a method loyalists did not learn until the end of the 1970s.

The expert's 100-page report submitted to the inquiry states:

"Loyalist terrorist groups did not have the skills to undertake this operation in 1974. Further, I do not believe they have ever possessed them, otherwise a similarly complex operation would have been repeated."

It goes on: "The loyalist terrorists who undertook this operation were at least guided, and very likely directed, by somebody with considerable knowledge of terrorist bombing activities. The most likely sort of person who could have provided that guidance is an ammunition technical officer or ammunition technician with experience of intelligence processes and practices and with access to loyalist terrorists."

The bomb disposal expert's report estimates that only five people, all of them British soldiers, combined this level of access to seized IRA explosives and to loyalist terror groups. It describes another bomb in Monaghan, near the border with Northern Ireland, which detonated 90 minutes after the Dublin blasts and killed eight people, as of standard loyalist construction.

It did not use IRA explosives and is believed to have been built by a different loyalist unit.

The bomb disposal expert's report was commissioned
by Justice for the Forgotten, an Irish government-funded group for the victims of the atrocities, at Barron's request in 2001. However, it is understood further reports have been commissioned from the same expert into other explosions in the republic. These found two other loyalist bombs were probably composed of explosives seized from the IRA.

One of these was detonated at Kay's Tavern in Dundalk, a bar which was at the time used by republicans, on December 19, 1975 and claimed two lives. The second exploded on the same day outside a bar in Silverbridge, Co Armagh, and killed three people.

Last month Bertie Ahern, the taoiseach, told the Dail that he had raised the alleged non-cooperation of British authorities with Paul Murphy, the Northern Ireland secretary, who assured him that "further material would be forthcoming".

Ahern added: "The prime minister is fully aware that we want to see the maximum co-operation from the British authorities and the matter has been raised through the British-Irish secretariat and at the recent meeting of the British-Irish intergovernmental conference."