A top republican who headed Sinn Fein's Stormont offices, tonight confirmed he was a British agent for two decades.
Denis Donaldson, a 55-year-old former head of administration at Stormont, said he was recruited in the 1980s as a paid agent.
Sinn Fein leader Gerry Adams announced today he expelled Mr Donaldson from the party after he admitted in an interview with Sinn Fein members that he worked for the British security services.
Mr Donaldson said he deeply regretted his activities.
In a statement to RTENews tonight, he said: "I was a British agent at the time. I was recruited in the 1980s after compromising myself during a vulnerable time in my life.
"Since then I have worked for British intelligence and the RUC/PSNI Special Branch. Over that period I was paid money."
Mr Donaldson was working as a Sinn Fein Assembly group administrator in Parliament Buildings during the PSNI raid on the party's offices in October 2002, which has since become known as the 'Stormontgate' affair.
Mr Donaldson echoed earlier comments from Mr Adams, insisting that 'Stormontgate' was a Special Branch "scam".
"I 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."
Earlier today Mr Adams, accompanied by north Belfast MLA Gerry Kelly, outlined the series of events that led to Mr Donaldson being expelled for "serious misconduct".
Mr Donaldson was approached by the PSNI last Saturday and warned that his life was at risk and that he was to be "outed" as a British agent.
He reported this to the chairman of the Sinn Féin six county cuig, Declan Kearney, and admitted after meetings on Tuesday and Wednesday at party offices on Belfast's Falls Road that he was a spy.
He was interviewed by Mr Kearney and Leo Green, a member of the Sinn Féin negotiating team, before being dismissed after consultation with party general secretary Mitchell McLoughlin.
Mr Adams said earlier he knew "instinctively ... there was somebody wrong" at the centre of the spy-ring allegations but did not particularly suspect Mr Donaldson.
Mr Adams speculated that the PSNI approach to Mr Donaldson was the initiation of plan to hinder the peace process. He said his "best guess" was that opponents of the process were going to suggest that Mr Donalsdon was under threat from Republicans after being outed as a spy.
With the Independent Monitoring Committee due to report in January, the "scandal" could have been used to send the peace process into another engineered "crisis".
"There was no spy-ring at Stormont. The fact is that that was a very carefully constructed lie which was created by the Special Branch to cause maximum political impact."
"It's obvious that people who become British agents, in most instances, are people who become blackmailed, bullied, coerced, bought, broken, used abused and then thrown to one side when they've had their use".
"What's very clear is that within the PSNI - and we've seen this recently - within the intelligence agencies - there are elements who are a law unto themselves. There's no accountability, there's no transparency, most people don't even know who they are."
He said Sinn Féin have been warning publicly and making representations to the British government about elements within the PSNI and intelligence agencies working to destroy the peace process.
"Here you have the stated policy of the two governments being subverted by agencies of one of those governments," Mr Adams said.
However, a statement released this evening by the Northern Ireland Office read: "We completely reject any allegation that the police operation in October 2002 was for any reason other than to prevent paramilitary intelligence gathering."
Arrangements have been made for him to speak to the Taoiseach and British Prime Minister by phone as soon as possible. Both Bertie Ahern and Tony Blair are at the EU summit in Brussels but Mr Adams is expected to talk to both men this evening.
"The fact is that the collapse of the political institutions was a direct result of the actions of some of those who run the the intelligence and policing system of the British," Mr Adams said.
He speculated that at least some of the intelligence allegedly gathered by Mr Donaldson's had been seen by Mr Blair. He said the Prime Minister must rein in the security agencies. "This is entirely the responsibility of the British government."