The verdict marks the end of a freedom of information battle between the PSNI and media outlet DeclassifiedUK.
It comes as a blow to legacy campaigners and bereaved families who hoped the report could shed light on alleged collusion between Special Branch and loyalist paramilitaries.
The dossier was drawn up by Jack Morton, a former MI5 director and colonial counter-insurgency veteran. He had been called out of retirement by the head of MI5 who picked him for "special duties in Northern Ireland".
Morton spent eight weeks in the summer of 1973 reviewing "the organisation, staffing and equipment" of RUC Special Branch in the fight against the IRA.
Less than a year after Morton completed his report, UVF car bombs killed 33 people and an unborn child in Dublin and Monaghan. No one has ever been charged with the atrocity.
The bombs are said to have been built at a farm in Glenanne, south Armagh, owned by an RUC reservist and used by a gang of loyalist paramilitaries, police and soldiers.
Dublin bomb campaigner Margaret Urwin from Justice for the Forgotten said: "It is of crucial importance for us to be able to access the Morton Report as we have long been aware of the close relationships between members of the Glenanne Gang and RUC Special Branch.
"As they have done with so many hundreds of other files on Northern Ireland, which have been closed for 84 years or 100 years, the British authorities have applied the national security blanket possibly in order to cover up human rights violations."
Her call was echoed by Kevin O'Loughlin, whose mother Christina was killed in the blast on Dublin's South Leinster Street, who said the Morton Report was "absolutely something we'd really like to know more about".
It's now feared the report may never be published after an Information Tribunal ruled that it had been supplied by MI5.
Under UK transparency laws, MI5 documents can stay secret for ever.
This is the first time the ruling, made in August, is being reported.
A PSNI spokesman said the force had "noted the decision of the Information Tribunal" and had "no further comment to make on this matter".
And a former head of RUC Special Branch, Raymond White, said this week that "there's nothing in terms of the Branch that we're afraid to discuss".
The retired assistant chief constable said collusion was a "toxic term" that "sticks like mud and people will interpret it in whatever way they want".
He believes Morton's report advised that agents and informants should be handled "in a professional way" by Special Branch.
"I joined the department in 1974 so it was in the Morton era," he said. "Whilst I never read the report I certainly was in the company of many [who had] and I would regard myself as a Morton recruit."
Mr White believes Morton "kickstarted" a thinking process within the RUC that led to "the birth of the armed uniformed units that were needed for quick reactions in terms of stopping vehicles that were bearing armed paramilitaries".
These squads would be guided to their targets by a surveillance unit within Special Branch known as E4A, which was later involved in several controversial shoot-to-kill incidents.
Morton served as a colonial police chief in India and was director of intelligence in Malaya during a communist insurgency against British rule in the 1950s.
His colonial background gave him extensive experience of undercover operations and armed policing.
MI5 did not respond to a request for comment.
Phil Miller is Declassified UK’s chief reporter.