In addition, the RUC may have kept information from gardaí investigating a bombing in order to hide security force collusion in attacks.

British Prime Minister Tony Blair was called upon last night to order the RUC’s successor, the PSNI, to cooperate fully with Oireachtas hearings that will be held on the issue later this year.

The findings were contained in an interim report of the commission investigating the bombing of Kay’s Tavern in Dundalk on December 19, 1975, in which two people were killed.

Later that same night, a gun and bomb attack was carried out at Donnelly’s Bar in Silverbridge, Co Armagh, killing three more people.

In his report published yesterday, former Supreme Court judge Henry Barron — the sole member of the commission — said the Dundalk bombing was carried out by loyalists, most probably associated with the mid-Ulster UVF.

These loyalists were using the farm of an RUC reserve member, James Mitchell, as their centre of operations, although Mr Justice Barron said he accepted the bomb did not originate from the farm, located in Glennane, Co Armagh.

However, he believed the Dundalk and Silverbridge bombings were coordinated by members of the “Glennane group”, and therefore “members of that group must at least have known in advance of the plan to attack Dundalk”.

The judge said allegations of collusion were “impossible to prove or disprove”. However, he could say that:

The Glennane group contained members of the RUC and the British Army’s Ulster Defence Regiment (UDR), “some of whom probably knew of the plan to attack Dundalk”.

Senior members of the security forces “allowed a climate to develop in which loyalist subversives could believe that they could attack with impunity”.

Some of those suspected of the bombings, such as well-known loyalists Robin Jackson and brothers James Nelson Young and Joseph Steward Young, had relationships with British intelligence and/or RUC special branch officers.

While the commission did not have firm evidence, Mr Justice Barron said he suspected the RUC kept crucial intelligence from the gardaí investigating the Dundalk attack in order to “limit information relating to security forces collusion in terrorist activity from reaching the public domain”.

The judge also said that, while forensic evidence was inconclusive, the nature of the explosives used suggested a possible link between the Dundalk attack and the bombings of Dublin and Monaghan in 1974 and Castleblayney in 1976.

An Oireachtas sub-committee will begin hearings on Mr Justice Barron’s report in September, and issue its own report by mid-November. But families of the victims again expressed disappointment last night that no public inquiry would be held.

Sinn Féin said the Taoiseach needed to hold a special summit with Mr Blair to discuss exclusively the issue of collusion.