The UK Government has won its landmark bid to secure a partially secret hearing in a legal action over intelligence gathered on the Omagh bombing.
A High Court judge in Belfast has granted a declaration being sought to enable a closed session, amid fears that the disclosure of sensitive material could damage national security.
The move forms part of a wider challenge to former Northern Ireland Secretary of State Theresa Villiers' refusal to hold a public inquiry into the August 1998 atrocity.
Michael Gallagher, whose son Aiden was among 29 people killed by the Real IRA bombers, is attempting to have her decision judicially reviewed.
The case centres on claims that a range of intelligence from British security agents, MI5 and RUC officers could have been drawn together to prevent the attack.
An alleged gap in the information relates to any monitoring of the bomb and scout cars as they crossed the Irish border into Omagh on the day of the outrage.
Counsel for the Secretary of State applied for the right to hold a closed materials hearing ahead of the main legal challenge getting underway.
That process, under powers contained in the Justice and Security Act 2013, examines whether public disclosure of some information could threaten national security.
Intelligence documents would be assessed by a judge and a special advocate barrister appointed to protect the rights of Mr Gallagher while he is shut out of proceedings.
But the first stage involved deciding if the case is suitable for closed materials.
The application is believed to be the first of its kind in judicial review proceedings in Northern Ireland.
Ahead of the hearing a sample of the closed material was gathered and stored at a secure location away from the Royal Courts of Justice.
Delivering judgement on the government's application today, Mr Justice Maguire held that sensitive material will be disclosed.
He also concluded it was in the interests of justice to make the declaration sought.
The judge said: "The court is satisfied that the sample of closed material which it has seen contains substantial reference to sensitive material, the disclosure of which would be damaging to the interests of national security."
"This material would, in the court's view, have to be disclosed by the Secretary of State in order both to make the defences it seeks to assert and to meet the requirements of candour which rest upon him or her for the purpose of the judicial review."
The case will now advance to a full hearing on dates to be listed early in the new year.
Mr Gallagher launched his legal action after Ms Villiers rejected calls for a public investigation in September 2013.
She decided instead that a probe by Police Ombudsman Michael Maguire was the best way to address any outstanding issues surrounding the atrocity.
In October 2014 Dr Maguire published a report where he found RUC Special Branch withheld some intelligence information from detectives hunting the bombers.
No one has ever been convicted of carrying out the attack.
Central to the judicial review challenge is a contention that the British Government has a duty under Article 2 of the European Convention on Human Rights to protect lives and investigate the bombing.