An MI5 report on policing compiled at the height of the Troubles will remain secret nearly half a century after it was written, Britain’s freedom of information watchdog has ruled.

The Information Commissioner’s Office (ICO) said police in the North do not have to disclose the so-called Morton report, which recommended a shake-up of RUC Special Branch in 1973.

In reaching its verdict, however, the ICO confirmed for the first time how significant the report could be for understanding the history of policing in the Troubles.

The ICO said the report was instigated by the head of MI5, Sir Michael Hanley, who suggested to then RUC chief constable Sir Graham Shillington that a senior security service officer conduct a review of Special Branch in June 1973.

The officer chosen was Jack Morton, a former colonial police chief in India.

Gallery Border Roads by Tony O’Shea VIEW NOW

A year after Morton wrote his report on Special Branch, the UVF killed 33 people and an unborn child in a series of car bombs across Dublin and Monaghan. It was the deadliest attack of the Troubles.

Barron inquiry

An inquiry by former Irish Supreme Court judge Henry Barron later concluded: “A number of those suspected for the bombings were reliably said to have had relationships with British intelligence and/or RUC Special Branch officers.”

Morton’s report on the Troubles is so sensitive that it took the ICO 18 months to reach a decision, in what became one of the watchdog’s top 10 longest-running cases.

More than 20 academics from Britain and Ireland had called for the Morton report to be released for their research, but to no avail.

This month, the commissioner ruled the report could stay secret after MI5 confirmed it had originally supplied the document to the RUC.

Daniel Holder, deputy director of Belfast-based NGO the Committee on the Administration of Justice said the decision was “difficult to conceive”.

“There is a right for such information to be put into the public domain,” Mr Holder said.

“The application of blanket national security vetoes over historic information is not human rights compliant,” he warned, “particularly when the documents in question may contain evidence of past security policies and practices that involved or facilitated human rights violations.”

Mr Holder said the report should be disclosed to prevent “a repetition of past practices that fuelled conflict”.