Senior Irish police ignored a clear warning about the 1998 Omagh bomb to protect an informer, according to a Sunday newspaper report.
A transcript of a conversation between the informer and his Garda police handler indicates that the bomb was allowed to 'go through', according to the Observer newspaper.
In the recording, made only days before the informer was forced to flee Ireland, he warns the Irish state that 'Omagh is going to blow up in their faces'.
The alleged informer was a car thief who infiltrated the dissident 'Real IRA' for the Garda.
The day before the Omagh bombing, the informer was in constant contact with his handler. The Garda police were aware of the breaker's yard where the car was fitted with reinforcements to carry the bomb, but it was eventually driven over the border.
In the transcript it is clear that Garda detectives let the bomb cross the border in order to give him credibility. The transcript contains no insight into the role of the RUC police once the bomb had crossed the border.
Last night relatives of those killed said they were stunned by the revelations. and called for a public inquiry on both sides of the Irish border.
The informer has never been offered to the PSNI for interview. It was claimed provided intelligence on nine different Real IRA attacks between February and August 1998, five of which were thwarted, but four were allowed to go ahead.
There had been no injuries in previous attacks, but 31 civilians died in the Omagh bombing, which led to the collapse of the 'Real IRA' amid a wave of public revulsion.
Some have suggested parallels to the Dublin and Monaghan bombings of 1974, in which British forces are alleged to have colluded with loyalists to incur massive loss of life in the South as a strategy to undermine support for the IRA.
Michael Gallagher, whose son Aidan was killed in the atrocity, said the victims' families had no faith in the Dublin-based inquiry. Instead he called for an international public inquiry to be held on both sides of the Irish border.
Gallagher said the families believed the new allegations. "The intelligence services in Dublin, notably some senior Garda officers, were playing Russian roulette in 1998".