Two bombs planted in the city’s Mulberry Bush and Tavern In The Town pubs exploded on November 21 1974, killing 21 people and injuring up to 220 more.
Nobody has ever been brought to justice for the killings.
In 2019, an inquest found a botched IRA warning was responsible for victims’ unlawful killings.
However, the coronial process was described as “unsatisfactory” by some of the bereaved, for not prompting criminal charges against any of the perpetrators.
KRW Law which represents 10 of the victim’s families, made a Freedom of Information Act request to the Northern Ireland Office in November 2017, which holds the file, referenced CJ4/6052.
The file – described as “Provisional IRA intentions and activities in Great Britain 1975 Jan 01 – 1975 Dec 31” – came to the Belfast-based law firm’s attention after its title was listed in a schedule put together during preparation for the now-completed inquest.
Lawyers for the firm previously said they wanted access to the file, collated in the year after the bombings, as it charted “political thinking and the reaction to the mainland terror campaign post the pub bombings”, claiming it was therefore of significant public interest.
In a statement on Wednesday, KRW said the Information Rights Tribunal had rejected its appeal against the original refusal of NIO to disclose the file, on national security grounds, in a ruling made on February 24.
Twelve pages from the file were previously released with redactions, shortly before a previous court hearing on the issue, in February 2020.
The appeal tribunal concluded that there continued to be a threat to national security in Northern Ireland from dissident Republicans and the threat had been continuous, from 1975.
Its ruling noted the file related to the mainland UK, which was outside the jurisdiction of the Northern-Ireland based data-holder, the NIO.
The tribunal also concluded the lawyers’ arguments the historic information would contribute to the fact-finding process in relation to the bombings, was unlikely because the file covered the IRA activities and intentions across the mainland UK.
Barry O’Donnell, KRW associate solicitor, said the firm would “continue to make” such requests for information, and “seek relevant material” on the legacy of the Troubles, for all those it represented.
He added: “We will continue to argue that national security cannot continue to be used to suppress information which would benefit relatives of victims and survivors of the conflict in Northern Ireland.
“If national security means anything it must mean protecting democracy and the rule of law, the foundations of which are transparency and accountability.”
In November 2020, a 65-year-old man was arrested in Belfast in connection with the bombings by officers from West Midlands Police assisted by the Police Service of Northern Ireland.
He was later released and the police investigation continues.