Denis Donaldson (55) was the Sinn Féin head of administration at the Northern Ireland Assembly at Stormont when he was arrested and charged with having documents likely to be of use to terrorists.
The west Belfast resident and one time friend of hunger striker Bobby Sands was a key aide to Gerry Adams who helped ensure Sinn Féin's Stormont machine ran smoothly.
Last night, Mr Donaldson said he had been a British agent for the last two decades, having compromised himself in the 1980s during what was a "vulnerable" period of his life.
"Since then I have worked for British intelligence and the RUC/PSNI Special Branch. Over that period I was paid money," Mr Donaldson told RTE.
He said he "was not involved in any republican spy ring in Stormont. The so-called Stormontgate affair was a scam and a fiction, it never existed, it was created by Special Branch.
"I deeply regret my activities with British intelligence and RUC PSNI special branch. I apologise to anyone who has suffered as a result of my activities as well as to my former comrades and especially to my family who have become victims in all of this."
Mr Donaldson came to public attention in October 2002 after the PSNI raided Sinn Féin's offices at Stormont as part of an investigation into republican intelligence-gathering.
His arrest, along with that of his son-in-law, Ciarán Kearney, and of William Mackessy, a former Stormont porter, became known as Stormontgate. Two days later he appeared in court on five charges, and exactly 10 days after the raid, devolution collapsed.
During a subsequent High Court bail application, it was claimed that he had risked his life to help free Beirut hostage Brian Keenan. Mr Keenan - held hostage in Lebanon between 1986 and 1990 - sent a letter of reference to the court.
It said Mr Donaldson had talks with an adviser to the Hizbullah group holding him. Mr Keenan stated: "For the whole period of my incarceration, only two human beings put their lives at risk on my behalf - one was Terry Waite and the other was Denis Donaldson."
Last week a surprise court hearing was told the Director of Public Prosecutions was not proceeding with the case against any of the men. No reason was given.
Mr Donaldson's lawyers said it was because they had been seeking documents from the crown relating to claims that the security services had a spying operation.
The day after the charges were dropped Mr Donaldson sat in a press conference at Stormont flanked by Gerry Adams and Martin McGuinness.
He insisted the spy-ring charges he had faced were politically inspired. He said: "There was no spy-ring at Stormont. There never was".