The Continuity IRA has denied that it cooperated with the 'Real IRA' in carrying out the Omagh bombing in August 1998.
The Special Criminal Court trial of Michael McKevitt in June heard a claim by FBI agent Mr David Rupert that both dissident groups acted together.
Mr Rupert said he was told by McKevitt that the Continuity were "80 per cent responsible" for the bombing, which killed 29 people and injured hundreds more.
The agent told the court McKevitt said the Continuity IRA had stolen the car the device was transported in and had driven it into the Co Tyrone town. The "Real IRA" had built the bomb itself.
However, the Continuity IRA said in statement today that it "totally refutes" the claim. It also says it regrets the "tragic events at Omagh".
"We wish to totally refute these absurd lies uttered by this mercenary and, for the record, we wish to state that we played absolutely no part in the preparation or carrying out of this atrocity."
The statement also described the McKevitt trial, after which he was sentenced to 20 years in prison for directing terrorism, as a "political show trial in the Stalinist style".