Lawyers for the Sinn Féin president Gerry Adams have formally contacted the BBC complaining about a Spotlight programme which alleged he sanctioned the 2006 murder of former senior party official and British agent Denis Donaldson.

The BBC confirmed on Thursday evening that it has received a letter from lawyers for Mr Adams.

"Lawyers will respond in due course," said a spokeswoman for the BBC, who added that "the Spotlight programme dealt with matters of great public interest and the BBC stands by its journalism".

A former Real IRA leader has also insisted that Mr Adams had nothing to do with the killing of Mr Donaldson.

Mr Adams made clear on Thursday that he intended to press a legal case against the BBC.

“I have been consulting with my lawyers and we will now be taking action against the BBC in relation to this totally false allegation contained within the BBC Spotlight broadcast,” he said, adding that he would not be making any further comment.

The BBC refused to say whether a writ has been issued. It is understood however that at this stage Mr Adams’s legal representatives are threatening to take legal action.

A Sinn Féin source said it was the beginning of a “legal process” against the BBC.

Mr Donaldson was shot dead near Glenties in Co Donegal in April 2006 after he confessed to being a British agent. The revelation directly led to the collapse of Stormont's institutions. His killing, which was claimed three years later by the Real IRA, has still not resulted in prosecutions.

British agent

The Spotlight programme based its claim on testimony provided by another British agent called "Martin" who had infiltrated the Provisional republican movement. "I know from my experience in the IRA that murders have to be approved by the leadership," he told Spotlight. Specifically asked to identify who ordered the killing, he said: "Gerry Adams, he gives the final say."

Mr Adams described the allegation as "nonsense". He told the Michael Reade Show on LMFM: "All I can do is tell you the truth of the matter is that there's nothing true about this story…I would like to think that a public service broadcaster like the BBC has standards. They will have to defend their actions."

The Belfast Telegraph quoted an ex-Real IRA leader who insisted that his former organisation and not the Provisional IRA murdered Mr Donaldson.

“Let me be clear about this. A claim of responsibility was made by (the Real IRA) in 2009 and it was correct,” he told journalist Suzanne Breen.

“Gerry Adams had absolutely nothing to do with the execution of British agent Denis Donaldson. The Provisional IRA wasn’t involved in any shape or form. I don’t know why allegations that the Provos did it are now being made but they are totally untrue,” he added.