Lawyers acting for Gerry Adams have written to the BBC over allegations about the murder of an MI5 spy within the IRA.
Denis Donaldson, 55, a Sinn Fein official and close colleague of Mr Adams, was shot dead at an isolated cottage in Co Donegal in 2006.
He had been living in the remote area following his exposure as a state agent the year before his death.
A recent BBC Spotlight programme alleged the Sinn Fein president had sanctioned the killing of Mr Donaldson.
The Spotlight allegation was made, on an anonymous basis, by a man who claimed he was also a paid state agent within the IRA.
Mr Adams, who has always denied he was in the IRA, branded the documentary “nonsense” and last week said he specifically and categorically denied the allegation that he had any involvement in ordering the murder.
Thursday’s edition of the Andersonstown News, covering west Belfast, reported the Sinn Fein president as saying: “I have been consulting with my lawyers and we will now be taking action against the BBC in relation to the totally false allegation contained within the BBC Spotlight broadcast.”
He told the paper he would not be making any further comment on the matter.
It is understood Mr Adams and his lawyers want to pursue the matter as far as possible but he declined to comment further when pressed if that would mean taking a lawsuit in the courts.
Mr Adams said last week that he would not be reluctant to sue if he was advised to.
A spokeswoman for the BBC said: “The Spotlight programme dealt with matters of great public interest and the BBC stands by its journalism.”
In relation to the Andersonstown News article on Thursday, the BBC said: “Since this morning the BBC has received a letter from Mr Adams’ lawyers and will respond in due course.”
The corporation has declined to comment on the contents of the letter.
Dissident republican group the Real IRA claimed responsibility for the killing in 2009 but the circumstances surrounding Mr Donaldson’s outing as a British agent and subsequent death have long been shrouded in mystery.
Mr Adams also accused anti-Sinn Fein elements within the British establishment of concocting the claims over who sanctioned the murder.
Ulster Unionist justice spokesman Doug Beattie said any legal challenge would result in the programme’s claims being subject to heavy scrutiny.
The Upper Bann MLA said: “If Gerry Adams does indeed seek to sue the BBC for libel, it will provide an opportunity for the claims made in the BBC Spotlight programme to be held up to public scrutiny.
“The BBC should now produce the evidence to the PSNI and allow them to investigate the case which needs to be subjected to transparent legal scrutiny. If ever a case could be said to be in the public interest, it surely is this one.”
Mr Beattie added: “Many people in Northern Ireland and further afield will look forward to seeing Gerry Adams in a witness box. If character evidence is permitted, it could be a very interesting case indeed.”