The two main opposition parties tonight called for a cross-Border inquiry into the Omagh bombing following revelations that MI5 withheld vital anti-terrorism intelligence.
The British domestic secret service agency received a tip-off that dissident republicans were targeting Omagh or Derry for a major bombing attack and used it to help to thwart a bomb plot at the time of the signing of the Belfast Agreement in April 1998.
But M15 kept the police in Northern Ireland in the dark, according to authoritative security sources, and failed to pass on the information even after Omagh bomb outrage five months later which killed 29 people.
The Labour Party and Fine Gael said it was time to listen to the relatives of the Omagh bombing, who have been campaigning for years to get the truth.
Fine Gael leader Enda Kenny said the allegation that the British Intelligence Services had prior knowledge of an attack in August 1998 which was not passed on to the police was very disturbing. "This adds to a long fist of unanswered questions about the handling of the Omagh atrocity. The Government should seek an immediate explanation from the British government."
"Of course, this does not shift the blame away from those cowards who planted the bomb that caused the deaths of 29 people. Hopefully, in time, all those responsible will be brought to justice.
"But it does ask questions about how intelligence was used and whether or not many of those lives could have been saved had information been shared, properly processed, and the warning signals acted upon."
M15 received the tip off from American FBI agent David Rupert, who was working undercover in the ranks of the "Real" IRA. He warned that Omagh or Derry - but most likely Omagh - was to be hit by a dissident republican unit based in Fermanagh and the Letterkenny area of Co Donegal.
Three dissident republicans were arrested and later released without charge at the time of the foiled April 1998 bomb plot. At the time, the Royal Ulster Constabulary was aware that a planned terrorist operation had been disrupted but according to security sources today, police found no trace on their records of any MI5 intelligence that Omagh, or Derry, was going to be a target.
Details emerged after inquiries in the United States where detectives investigating Omagh spoke with Rupert and examined emails which the agent once exchanged with his FBI and MI5 handlers.
Fine Gael justice spokesman Jim O'Keefe said the failure by M15 to pass on the information reinforced the need for a cross border inquiry into the Omagh bombing.
"I'm very surprised and disappointed to learn the situation could possibly have led to the prevention of this appalling murder," he said. "Even at this stage, explanations are called for about how this happened."
"Today's revelations merely underline the legitimacy of their calls for a full, independent cross-border inquiry," said Labour's justice spokesman Joe Costello TD. He said the way M15 handled information in the lead-up to Omagh suggested people could have little confidence in their ability to prevent major atrocities.
Relatives of some of the Omagh dead said today they were astonished by the disclosure. Michael Gallagher, whose son Aidan (21) was among those killed, said: "At best, this is criminal negligence. At worst, it's assisting a terrorist murder plot."
MI5 will next year take control of national security intelligence in Northern Ireland. SDLP leader Mark Durkan called for the plan to be abandoned. "Allowing MI5 to have a lead role in intelligence in Northern Ireland would be like appointing Herod as children's commissioner," he said.