The watchdog investigated on the foot of complaints from the men, three of whom were aged 17 at the time and one of whom was aged 18, alleging that the confessions were fabricated and that the interviewing officers had perverted the course of justice.

Marie Anderson, the police ombudsman, said the men had been subjected to a “coercive and oppressive atmosphere” and were not given the opportunity to have legal representation before signing a total of 21 “confessional” statements.

The statements related to five terrorist incidents including the murder of a 22-year-old soldier, Lieutenant Stephen Kirby, who was shot dead by the Provisional IRA at Carlisle Terrace in Derry on 14 February 1979.

The men – Michael Toner, Stephen Crumlish, Gerard Kelly and Gerald McGowan – also signed statements admitting involvement in a number of punishment shootings in the city in January and February 1979. They have always protested their innocence and in December 1998 were acquitted by the former Lord Chief Justice and found not guilty of all charges.

Following their alleged confessions, the Director of Public Prosecutions directed that the four men should be prosecuted. On the third day of their trial in October 1980, all four failed to appear, having absconded to the Republic of Ireland. They later stated that they did not believe they would receive a fair trial.

After absconding, the men were arrested by police in the Republic of Ireland, but were later released when no extradition requests were made by the RUC. They remained south of the border until the DPP withdrew the charges against them in December 1998.

The men each stated that police actions had deprived them of a family life as they had been unable to attend important family events such as weddings and funerals during this period.

In 2003, they made complaints to the Police Ombudsman’s Office alleging that they were subjected to mental and physical abuse during their time in custody. The watchdog published the 118-page report of its investigation on Friday.

Following the investigation, Mrs Anderson said: “Taken together, the prolonged and repeated nature of the interviewing, the immature ages of the four young men, their inexperience with law enforcement, and the absence of access to legal advice or other support made them susceptible to compliance with those in authority.

“I am of the view that these factors had the cumulative effect of creating an oppressive and fearful environment in which they made ‘confessional’ statements. It is my view that the ‘confessional’ statements were not obtained fairly, but by coercion and/or oppression.”