A senior figure in Sinn Fein who was exposed last December as a British agent has been found shot dead in Ireland.

Irish police said the body of Denis Donaldson had been found just before 5pm near the cottage in Donegal, where he had sought refuge four months ago after being outed by senior republicans.

Two weeks ago, an Irish tabloid newspaper discovered Mr Donaldson, 56, living in the remote cottage near the village of Glenties with no running water or electricity.

The area was sealed as forensic experts and a pathologist were called in to carry out inspections and a post-mortem examination.

The Irish Government swiftly condemned the killing, apparently ruling out the possibility that Mr Donaldson had committed suicide.

"We condemn this brutal murder," said Bertie Ahern, the Irish Prime Minister. "The matter is now under investigation by An Garda Siochana. We hope that whoever was responsible for this callous act will be brought to justice as soon as possible."

Gerry Adams, the Sinn Fein leader, with whom Mr Donaldson once shared a cell, said tonight that that he disassociated Sinn Fein from the killing, if it was murder. A British Government spokesman said that the Government "condemned the murder as much as any other" and noted that Mr Adams had done likewise.

Mr Donaldson, a convicted bomber, was named as a spy at a press conference last December. He confessed the same night that he had informed on the republican movement for more 20 years. His exposure as a double agent came after he was arrested, with his son-in-law, Ciaran Kearney, as one of the central figures in the alleged IRA "spy-ring" that led to the suspension of the power sharing assembly at Stormont in late 2002.

Mr Donaldson, then chief of staff for Sinn Fein at the assembly, was accused by the British security services of helping to steal more than 1,500 pages of sensitive documents, including the names and addresses of thousands of staff in the Prison Service, which led to the relocation of 2,000 people at a cost of £300 million.

All charges against Mr Donaldson were dropped last year when the Director of Public Prosecutions in Northern Ireland said the case was no longer "in the public interest".

In December, Mr Donaldson said that he deeply regretted his life as a double agent, saying: "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."

He also said that the "Stormontgate" scandal had been an invention to save the Ulster Unionist leader David Trimble, the former First Minister who was losing control of the power-sharing executive when news of the alleged spy-ring broke.

The first agent exposed since the official end of the IRA's armed campaign, Mr Donaldson was thought to have been guaranteed his life by fellow republicans. During the Troubles, scores of republicans found to be in the pay of the British were shot in the back of the head and left by roadsides.

When he was tracked down at his lonely cottage two weeks ago, Mr Donaldson only answered a few questions from a Sunday World reporter. Asked about his dismissal from Sinn Fein, he said: "I don’t want to be in touch with anyone. As you can see, I’m in the middle of nowhere."

When asked what his future held, Mr Donaldson replied: "This is it."

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