Police in the Republic of Ireland confirmed they were investigating the discovery of a man in his mid-50s in the northwestern county of Donegal.

"The State Pathologist has been contacted and the scene is being preserved pending the arrival of the Garda Technical Team," the police said in a statement.

They did not name the man but a police source confirmed local media reports that it was Denis Donaldson who last year was expelled from Sinn Fein, which wants to end British rule in Northern Ireland, after he admitted spying for London.

"I am satisfied that it is who it is reported to be," the source said, adding that it was too early to say whether he had been killed or taken his own life.

The Irish government issued a statement describing it as a "brutal murder". "The matter is now under investigation. We hope that whoever is responsible for this callous act will be brought to justice as soon as possible."

A spokesman for the Northern Ireland office said the British-ruled province's secretary of state, Peter Hain, was "appalled by this barbaric act" but declined to say if that meant he was treating the death as murder rather than suicide.

Donaldson was a convicted Irish Republican Army bomber who spent time in prison with Gerry Adams, now leader of the guerrilla group's political ally Sinn Fein.

Donaldson was again arrested in 2002 and accused of spying for Sinn Fein at the Stormont parliament in Belfast but in a dramatic twist he was expelled from the party in December after admitting he had been a mole for the British for two decades.

The IRA took the unusual step of issuing a brief statement: "The IRA had no involvement whatsoever in the death of Denis Donaldson."

The IRA called a ceasefire in 1997 and pledged last July to down arms. An independent watchdog reported in October that the guerrilla group was keeping to its pledge.

British Prime Minister Tony Blair and his Irish counterpart, Bertie Ahern, are both expected in Northern Ireland this week to unveil their latest plan to kick-start the province's mothballed assembly which collapsed in part due to allegations that Donaldson and others had been spying for Sinn Fein.

The Stormont assembly, in which Catholic Nationalist and pro-British Protestant parties on either side of the Northern Ireland's sectarian divide shared power, collapsed three years ago after a police raid on Sinn Fein offices.

(Additional reporting by Michael Smith in Dublin and Adrian Croft in London)