A Republican mural is seen on the side of a house in the Bogside are of Londonderry Christopher Furlong/Getty Images

A notorious supergrass who infiltrated the IRA in the early 1980s has been found dead at his home in Kent. Raymond Gilmour, from Londonderry, was a police informer at the height of the Troubles in Northern Ireland after becoming a Royal Ulster Constabulary (RUC) Special Branch informer when he was 17.

Gilmour lived under a pseudonym for more than 30 years after he was resettled in England by MI5 following the collapse in the trial of 35 IRA suspects in 1984. The 57-year-old was the only witness to the trial, which saw mass protests outside and the now razed court.

Part of his relocation meant that he had to leave his wife Lorraine and their two children in Derry, most of whom disowned him. It is understood that Kent Police are not treating his death as suspicious and an autopsy is due to take place.

He joined the Irish National Liberation Army (INLA) in 1976, as a 17-year old police agent working for the RUC before he moved onto the IRA in 1980. Then in 1982 his cover was blown when police used his intelligence to recover a machine gun.

The Belfast Telegraph reported that he had suffered from alcoholism in later life and had serious psychological problems. They added that he married twice again in England, but both relationships broke down.

They also reported that his decomposing body was found by his 18-year-old son from his second marriage on Friday 28 October and he was on disability benefits at the time of his death.

Gilmour was put into a witness protection programme after testifying against alleged Republicans in 1984. The case collapsed when the then lord chief justice, Lord Lowry, dismissed his evidence, saying it was "unworthy of belief".

Described as "show trials" by some it was claimed informers were offered cash and that secret deals were struck at a political level before the men and women were tried. The trials were, at the time, the largest in British criminal history and in one of them, in 1983, 22 IRA suspects were jailed for a total of 4,000 years.

His friend and fellow agent, Martin McGartland, said to the newspaper: "It is disgraceful that Ray died in these circumstances. He spent years begging MI5 for financial and psychological help. Instead, they turned their back on him. He was a broken man, a wreck of a human being, and they left him to die in the gutter.

"As far as I'm concerned, the security services have Ray's blood on their hands. They had plenty of opportunities to save him but they turned their back on him. He gave his all to them to help defeat the IRA but, when they had no use for him, they discarded him. He was treated like a third-class citizen."