One of Britain's most important informers inside the IRA has warned new agents who are asked to penetrate terrorist organisations to ensure that they get a contract from their handlers to secure their future.

Hunted for the last 25 years by ex-comrades in the Derry IRA, Raymond Gilmour suffers from post-traumatic stress disorder and a heart condition. Now virtually penniless and cut off by his former handlers, Gilmour spoke to The Observer this weekend about his sense of betrayal and the need for future agents to ensure that they are looked after for the rest of their lives.

Speaking from a secret location in England, Gilmour said: 'If I was a young kid, say from the Muslim community in Britain, thinking of working for the state inside one of the Islamic terrorist groups I would make sure I obtained a contract. I would ask for a legal document ensuring that, given the risks I was about to take, the state looked after me for the rest of my life. Because what happened to me would discourage anyone from signing up as an agent.'

From the age of 16, Gilmour served the state as an informer, first inside the Derry Brigade of the Irish National Liberation Army and later the Provisional IRA in the same city. In 1983 he gave evidence against dozens of alleged Derry IRA members as a supergrass, although the courts later dismissed his testimony as unreliable.

The RUC Special Branch officer who recruited him, Alan Barker, wrote in his memoir Shadows that Gilmour was arguably the most important informer the security forces ran inside the IRA within Derry.

Living under a false identity for 25 years and cut off permanently from his large family back in Derry, Gilmour contacted The Observer because 'I feel an enormous sense of betrayal by some of those I worked for'.

He said: 'The people I have dealt with over the last few years I assume are from MI5, and they basically don't give a shit about me. A couple of years ago they stopped paying for a psychiatrist whom I had been seeing for nine years, a counsellor that helped me deal with the flashbacks and the post-traumatic stress I have had to suffer.

'I feel as if I've been discarded. As well as the mental anguish, I have a serious heart condition and underwent major heart surgery a few years ago. I live on £177 per fortnight and I am only 47.

'Even my old RUC contacts have stopped returning calls. It's as if I have been blackballed. You risk your life to stop others getting murdered and maimed, but in the end you are forgotten about. You are an embarrassment from the past; you are no longer useful.

'I always thought that, once my career as an agent was over, I would be looked after for the rest of my life. I was an agent of the state, but the state doesn't want to know. That is a dangerous thing for the recruitment of future agents. Anyone thinking of infiltrating or working on the inside to bring down terrorist organisations should get it nailed down, on paper, legally binding, that they will receive a pension long after they have retired from this deadly game.'

Like many of the Irish republican informers who worked for Britain, Gilmour has suffered from depression, ill-health and alcoholism.

'You have to look over your shoulder for the rest of your life. There are times when I have been on holiday abroad where I have panicked, sensing that I saw old IRA men I worked with. It never goes away and no matter what people in Sinn Féin say, about informers being safe to go home, that is a lot of rubbish.

'Having said all that, I have absolutely no regrets about undermining the INLA and the IRA in Derry. They were murderers and they had to be stopped. There are dozens upon dozens of people in Northern Ireland today who are alive because of what I and other agents did. That was why it was worth all the hassle and the danger - lives were saved, they were stopped.'

However, the former supergrass said he was prepared to 'fade away' if the security services help him out.

'I am not asking for a house, all I want is a pension. But when I first started spying on the INLA, and later the IRA, I honestly believed that one day the Queen would give me a medal.'