An ex-IRA informer has asked the north's first minister, Peter Robinson, to find out why he has been refused a pension for his services to the British state.

Martin McGartland, who lives on £89 a week disability benefit at a secret location in Britain, was told MI5 had approved a pension for him. McGartland (39) was to be paid £350,000 over 26 years.

Details of the pension plan were sent to the Northern Ireland Office (NIO) and British Treasury solicitors. "In May 2008, my MI5 case worker told me that the NIO and British Treasury had also agreed to the pension. I was delighted," McGartland said.

"But a few weeks later, I was told there would be no pension. No reason was given. I suspect Sinn Féin heard about the pension and intervened to stop it. Sinn Féin is now well established in government – it has clout. I've written countless letters to MI5 asking for an explanation, but they won't reply."

McGartland was recruited by Special Branch to spy on republicans when he was 17. His handlers later asked him to infiltrate the IRA. His cover was blown in 1991. He was taken from Sinn Féin's Belfast headquarters to Twinbrook by Paul 'Chico' Hamilton, Gerry Adams' bodyguard, and Jim McCarthy, Adams' driver.

He was tied up and believed he was about to be killed. He escaped by jumping from a third-floor window, suffering serious injuries. He was again nearly killed by an IRA gunman who tracked him down in England in 1999. He was shot seven times.

The attack left him with a disability and unable to work. He suffers post-traumatic stress disorder. McGartland said: "When I was shot, I'd started a small property development business. I was only 29 and had good earning potential.

"Had a police officer or soldier been injured, they'd be entitled to a pension, so should I. After the shooting, my partner Jo had to give up work to look after me. She now receives £53 a week carer's allowance. If I hadn't been shot, we'd today be earning at least £50,000 a year between us. Instead, we survive on a pittance."

McGartland said Freddie Scappaticci, an informer who ran the IRA's internal security department, and some other informers, were receiving pensions.

"I don't know why Freddie Scappaticci gets one and I don't. He wasn't kidnapped. He wasn't shot. Freddie Scappaticci was fit and well when he left Northern Ireland. Indeed, he was given a free run out by Sinn Féin and the IRA."