Sinn Féin has expelled its former head of administration at Stormont, Denis Donaldson.
The party said Mr Donaldson was working as a British agent.
Last week the Director of Public Prosecutions in Northern Ireland dropped all charges against Mr Donaldson and two other Sinn Féin members who were accused of running a republican spy ring at Stormont.
The three were arrested and charged after police seized documents and computer discs in a raid on Sinn Féin's offices at Stormont on 5 October 2002.
The British government then suspended devolution, embarking on three years of direct rule, after the DUP and the Ulster Unionists threatened to collapse the Executive with resignations.
In a statement to RTÉ News this evening, Mr Donaldson confirmed that he has spied for British intelligence since the 1980s and he described the Stormont spying allegations as a sham.
Mr Donaldson said he deeply regretted his activities with British Intelligence and RUC/PSNI Special Branch and he apologised to anyone who has suffered as a result of his activities.
Sinn Féin President Gerry Adams said Mr Donaldson had earlier confirmed to two senior party members that he had been working as a British spy for the past 20 years.
According to Mr Adams, the PSNI told Mr Donaldson that he was going to be outed as an informer and that his life was in danger.
At that stage Mr Donaldson informed senior Sinn Féin members in Belfast of the threat.
The Taoiseach, Bertie Ahern, has said that the revelations concerning Mr Donaldson were 'as bizarre as it gets'.
Speaking in Brussels, where he is attending the EU summit, Mr Ahern said he hopes to talk to all the relevant parties to get as much information as possible on the matter.