There was a high alert inside the prison, after 40 sticks of gelignite and detonators were discovered in cells, on July 3 – and a planned breakout and mass murder attempt was foiled.

A Northern Ireland Prisons Situation Report, dated August 1977, and copied to the Secretary of State Roy Mason detailed the situation – along with letters and memos prepared by the prison governors.

It now emerges from the secret files that prison officers were tipped off on the escape plan by a prisoner.

The informer’s name is blacked out in the files.

Governor McMullan reported: “The prisoner has given information before but it has not always proved to be reliable.

“He is a scheming individual, who claims not to be involved with paramilitary types. His information is never stated as clear fact but is garbled and has to be sifted before action can be taken.”

The prisoner had overheard conversations about the breakout plan and the explosives in A and C Wings. Some 29 prisoners were involved.

Searches discovered the explosives, which governors believed were smuggled in by family in friends, inside food parcels – to remand prisoners, still awaiting trial, but in the republican cells.

The authorities immediately put a ban on the following items being brought into the jail by visitors: butter, margarine, tubes of toothpaste and chicken which contained bones.

Mr McMullan told the Secretary of State that he believed this was now “a golden opportunity” to crack down and “change direction” on prison rules, related to parcels – and nothing but clothes should be permitted.

“The adverse publicity will present problems,” he noted.

But this, he added, could be countered by the escape plan being fresh in people’s minds “if we act quickly” and by stocking the Crumlin Road tuck shop with all the items normally brought in by visitors.

He also said prison staff were now concerned for their safety, with canteen staff “concerned about cutting butter for fear of a concealed explosive device and detonating it”.