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Eamon Collins, who described his experiences in the IRA in his 1996 book Killing Rage, was found dead with severe head injuries at Doran's Hill in Newry, Co Down.

It was first thought that he had died in a hit-and-run accident but police said other theories are now also being investigated. A post mortem examination is being carried out on the body.

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Mr Collins, who confessed to involvement in a string of IRA murders in the 80s, had said he had been the victim of a hit-and-run two years ago. He rose to prominence after the publication of his book and wrote numerous articles for national newspapers.

After becoming increasingly critical of republican violence he spoke of a long series of intimidation.

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During a debate in the House of Commons on paramilitary "punishment beatings", the Ulster Unionist leader, David Trimble, said that Mr Collins had angered the IRA by criticising republican policy extremism.

He said that his home had burned down in suspicious circumstances but would not comment any further as police investigations are still under way.

Neil Belton, an editor at Mr Collins' publishers, Granta, said: "He was almost a recklessly courageous individual who seemed to have found some way of publicly atoning for what he had done through the book. It is a tragic, tragic waste."

Prepared assassinations

Mr Collins, a former IRA "intelligence officer", collected information on the RUC and Special Branch and set up assassinations over a six-year period.

Although he never directly shot anyone, he provided information, recruited members and learned the secrets of the organisation. He was subsequently charged with five murders.

But he was cast out of the movement after turning supergrass, even though he later retracted his evidence against his comrades.

IRA fury

In his autobiography, Killing Rage, he gives an inside account of the IRA. But he enfuriated the IRA by describing it as a chaotic, anarchic organisation whose members were happiest when they were in the pub.

Mr Collins had said he had renounced violence and deeply regretted the deaths of the people in which he played a part, including fellow Customs officer Ivan Toombs in 1981.

In his book, he describes how an undercover British Army officer was tortured and murdered in 1977 and how his body was disposed of in a meat mincer.

SAS undercover officer Captain Robert Nairac was abducted from a pub in South Armagh but his body was never found. Six republicans were convicted of offences connected to the murder.

Mr Collins said he was told the details while he was on the run in the early 1980s.

After his arrest by the RUC in 1985 he turned supergrass.

More than 40 suspects were arrested but most were released after his change of heart under pressure from his family.

He was charged with 50 terrorist offences including the five murders and membership of the IRA.

But he walked free from Belfast Crown Court after the judge dismissed his alleged confessions.

'I am not afraid of them'

After his release he said he was debriefed by the IRA for three months and given an "execution notice" warning that he would be killed if he stayed at home.

But the married father of four remained in his home town of Newry, Co Down.

Asked once if he thought the IRA would kill him, he said: "Well, I think about it now and again, but I don't let it dominate my life.

"No I'm not afraid of them. I don't want to die, but at the same time I've come close to death on many occasions.