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The British authorities covered up a case of child sex abuse by a senior figure of the IRA in order to use the damning evidence to turn him into a double agent, military intelligence sources have said.

Joe Cahill, who helped found the Provisional IRA, was pictured abusing a 14-year-old girl in a car in the 1970s, the sources said.

But instead of being prosecuted, the images were used by military spooks to turn the Republican hero – and close ally of Sinn Fein president Gerry Adams – into one of the British Army’s most valuable informants.

A source revealed that Cahill – who died in 2004 – was being followed by a covert unit as he drove around Belfast when the pictures were taken.

They apparently showed the paramilitary, then in his 50s, abusing a 14-year-old girl, who was later identified.

(Image: PA)

The source said: “The pictures clearly identified both Cahill and his victim. Her father would have killed him if he had found out. He was never prosecuted and instead the pictures were used to turn him. He was a prized asset.”

The allegation is the latest paedophile scandal to hit the IRA and Sinn Fein.

Last week Cahill’s grand-niece, Mairia, revealed the IRA covered up the alleged abuse she suffered as a 16-year-old at the hands of another IRA figure. Mairia was not Cahill’s alleged victim in the 70s.

The Ministry of Defence did not comment on our story last night.

But a military source said: “This kind of thing has been unthinkable for many years now. There were some very questionable techniques deployed in the 1970s but they were put a stop to very soon afterwards.”

Several secretive military intelligence units operated in Northern Ireland at the time, including 14th Intelligence Company, also known as the Det, and several special collation teams.

Female operatives were said to have slept with IRA figures for information.

Cahill’s role as a key British agent was known to only a small handful of individuals. But a senior IRA source revealed that there was suspicion that he was a “tout” – slang for informer.

The IRA’s chief of staff was a priceless source of intelligence as the terror outfit stepped up its bombing campaign across Ireland and the mainland.

He was responsible for arming the paramilitaries with weapons and explosives from allies such as Libya and raising money from republican sympathisers.

A high-level agent like Cahill could be used to save lives and also to exert pressure on the IRA and Sinn Fein to join the peace process, which took place in the 90s.

In a separate case, Mairia Cahill last week waived her right to anonymity to speak out against alleged sexual abuse she suffered as a 16-year-old over a year in 1997 at the hands of another senior IRA figure.

Ms Cahill said she was raped and later interrogated by the IRA about her claims. She later went to the police and a case was brought against the alleged rapist and those said to have been involved in the IRA inquiry.

The man she accused, Martin Morris, has consistently denied her claims and was acquitted of all charges, which were dropped after Ms Cahill withdrew her evidence.

She said: “The only word I have for it is interrogation, because that’s exactly how it felt. They told me that they were going to read my body language to see who was telling the truth and that they were going to bring him into a room.”

Gerry Adams has denied telling Mairia that sometimes abuse victims “actually enjoy it”.

Last year he came under fire after his brother Liam was jailed for sexually abusing his own daughter.

The 59-year-old is currently serving a 16-year prison sentence for raping and abusing Aine Dahlstrom when she was aged between four and nine in the late 70s and early 80s.

During a first trial which collapsed, Mr Adams told the court he confronted his brother about the allegations in 1987 and Liam denied the abuse.

He then revealed his brother later confessed to him in 2000.

Liam’s conviction led to pressure on his brother to explain why he did not contact police over the abuse allegations when he first learnt of them.

Gerry Adams told the first trial in April last year, that he warned a priest, who is now dead, about his brother’s sinister past and the pair became estranged after the allegations emerged.

Recent allegations of historical child abuse against political figures has forced the British Government to set up an official inquiry.

(Image: Phoenix Photo Agency)

Chairwoman Fiona Woolf is expected to examine abuse by MPs, in care homes and schools, and is now facing calls to expand her investigation to cover the role of the intelligence services.

Labour MP Tom Watson said: “There have been rumours that the intelligence services had knowledge of child abuse for a number of years.

"That is why it’s essential that the inquiry is given full access to intelligence service files on this subject.”

Labour MP Simon Danczuk, who exposed the truth about paedophile Liberal MP Cyril Smith, said: “The exploitation of children by the secret services and members of the establishment comes as no surprise.

"We are discovering a history of poor children being treated like disposable goods by people in high places and no one batted an eyelid.”

When approached about our story regarding Joe Cahill, a Sinn Fein spokesperson said: “This is contemptible. It is gutter journalism of the worst kind.”