A former driver for Sinn Féin president Gerry Adams worked as a British spy, it was disclosed tonight.
Roy McShane was once part of the so-called security team which looked after transport arrangements for the republican leadership.
Sinn Sinn Féin admitted he worked for the intelligence services for a number of years and earlier today had left his home in west Belfast.
Police tonight refused to comment on the disclosure, which will be deeply embarrassing for Mr Adams after the sensational disclosure two years ago that one of his closest administrative aides also worked for MI5.
Denis Donaldson (56) who headed Sinn Féin's support team at Stormont, was later shot dead near Glenties in Co Donegal.
McShane, a former IRA man from the Clonard area of Belfast's lower Falls, was once a driver for senior republicans, including around the time of the developing peace process which led to the signing of the Belfast Agreement ten years ago.
Even though he would have been considered no more than a rank and file republican - he was described tonight by former associates as a "working class foot soldier" - he would have been privy to highly sensitive information as he ferried senior republicans back and forward to Castle Buildings where the intense negotiations took place.
He would have been on first name terms with all the top men and women in Sinn Féin, including Martin McGuinness, now deputy First Minister in the Northern Ireland Assembly.
Donaldson was shot dead at an isolated cottage five months after he admitted working as a double-agent, but republicans tonight said Mr McShane had nothing to fear if he returned home.
They claimed he had been under suspicion for a number of years. A Sinn Féin spokesman said: "He is under no threat from republicans. If he wishes to return, it is up to him to make peace with his community and in particular his family."
McShane would have worked closely with the Sinn Féin team of bodyguards, some of whom carried guns, who protected Mr Adams and Mr McGuinness and would have been one of MI5's top informers, reporting regularly to his handlers and getting paid for information.
PA