He has called on the Irish government and the UK government to investigate.
A high court judge in Belfast has recommended the UK government undertake a human rights compliant investigation into the Omagh bombing, and has also encouraged the Irish government to do likewise.
Mr Justice Horner said it was plausible that there was a "real prospect" the 1998 Omagh bombing could have been prevented by the security services.
The ruling was made at Belfast High Court in a case brought by Michael Gallagher, whose son Aiden was one of the victims of the bombing by the Real IRA.
In his ruling, the judge said: "I am satisfied that certain grounds when considered separately or together give rise to plausible allegations that there was a real prospect of preventing the Omagh bombing.
"These grounds involve, inter alia, the consideration of terrorist activity on both sides of the border by prominent dissident terrorist republicans leading up to the Omagh bomb."
“I am therefore satisfied that the threshold under article 2 ECHR [European Convention on Human Rights] to require the investigation of those allegations has been reached.”
He added: "It is not within my power to order any type of investigation to take place in the Republic of Ireland but there is a real advantage in an Article 2 compliant investigation proceeding in the Republic of Ireland simultaneously with one in Northern Ireland.
"Any investigation will have to look specifically at the issue of whether a more proactive campaign of disruption, especially if co-ordinated north and south of the border, had a real prospect of preventing the Omagh bombing, and whether, without the benefit of hindsight, the potential advantages of taking a much more aggressive approach towards the suspected terrorists outweighed the potential disadvantages inherent in such an approach."
The bombing in August 1998 killed 29 people, including a woman pregnant with twins, and is regarded as the worst single atrocity of the Northern Ireland conflict.
No one has ever been convicted of carrying out the attack.
