IRA informer Raymond Gilmour has been found dead in his flat in Kent, where he had been lying abandoned and alone, for up to a week. His body was so badly decomposed, he had to be identified from photographs.
An autopsy is being carried out but friends believe that Gilmour (55), who was an alcoholic with serious psychological problems, died from natural causes.
He was on disability benefit at the time of his death.
His funeral will take place next week. It is not known whether he will be buried under his real name, or the pseudonym under which he has lived in Britain since he fled his native Derry over 30 years ago.
His friend and fellow agent, Martin McGartland, last night said: "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."
Gilmour gave evidence against 31 men and women in one of Northern Ireland's best known republican supergrass trials. After the case collapsed in 1984, he was resettled in England by MI5 and given a new identity.
But he never got over leaving his family in Derry, most of whom disowned him.
He had no contact with his wife Lorraine nor their two children. He suffered from alcoholism and serious psychological problems. He married twice again in England, but both relationships broke down.
He increasingly cut a lonely figure in the small seaside town where he lived. Sometimes, he ended up in a police cell, after becoming involved in fights with local men. Other times, he lived like a hermit, spending weeks drinking in his flat, refusing to go out or talk to anybody.
His body was found by his 18-year-old son from his second marriage, whom the Belfast Telegraph is not naming.
"'A' had been ringing his dad for about a week but he didn't answer. At the start, he wasn't overly worried because that was normal behaviour for Ray. He would drink and shut himself off from the world," Martin McGartland explained.
"But as the days went on, his son grew concerned and called in on Ray on Friday. The moment he entered the flat, he knew something was wrong. He was hit by an overpowering smell. His daddy's body had lain so long, it had started to decompose.
"Ray was unrecognisable. His son rang 999 and the paramedics who arrived wouldn't let other relatives see Ray because they said it would be too disturbing. They also advised against seeing him in the morgue."
McGartland said he feared that Gilmour would be given a pauper's funeral.
"Ray was living on peanuts when I met him. At times I had to put money into his bank account. Given the lives that he saved for the British state, he should be buried like a serviceman. I'll pay for a decent funeral for him myself if need be," the former agent said.
McGartland said he hadn't spoken to Gilmour in recent times.
"Ray was very hard work. I've my own problems and dealing with him was just too much. During phone conversations, he would threaten to take his own life.
"He wasn't getting the treatment he needed for his psychological problems. He was referred to his local NHS, but they wrote to Andrew Parker, the head of MI5, saying they couldn't deal with such big issues from Ray's Northern Ireland days. I also asked MI5 to help him many times but got nowhere," he said.
McGartland described how Gilmour went on a downward spiral. "On a good day, he was a lovely bloke with a heart of gold. But things took their toll on him. He wasn't as resilient as I am in dealing with problems. I hope his death is a wake-up call to the security services to start treating their former agents as human beings," he added.
Gilmour never regretted becoming an informer, his friend claimed. "He was immensely proud of what he did and believed it was the right thing to do," he said.
"But I would caution any young men and women in Northern Ireland thinking of becoming agents for the security services. They should look at how Ray Gilmour ended up, before they make their decision."