The DUP Upper Bann MP said he is prepared to use parliamentary privilege to reveal the identity of the senior Sinn Fein politician who, he claims, allegedly avoided charges in relation to the murder of an RUC man in 1979 by becoming a British spy.

However, Sinn Fein hit back describing Mr Simpson's suggestions as "baseless" and accusing him "like other DUP colleagues" of lacking "moral courage" by " hiding behind parliamentary privilege".

A spokesman said: " I suspect like his DUP colleagues he won't have the courage to make similar baseless allegations without parliamentary privilege. People should judge Mr Simpson by his motives."

Mr Simpson plans to name the person who believed to have planned the murder of his cousin - father-of-two Frederick 'Eric' Lutton, a former RUC reservist - while two other republicans shot him dead on May 1, 1979, near Moy in Co Armagh.

Mr Lutton was a caretaker for the National Trust and was killed as he left his car to lock the gates of its premises in Moy.

His son Nigel Lutton said that using the House of Commons to make public allegations surrounding the killing was justified as "a means of re-igniting an investigation" .

He said that if the person in question has nothing to hide then they should have an opportunity to clear their name.

"If this person was involved, and I believe that is the case then it (the naming) has to happen.

"Gerry Adams is calling for the truth. Well I say 'let's have it'. Let's have Sinn Fein's truth.

"My aim in all of this is simply to get the truth about who killed my father and to see prosecutions," he told the News Letter.

Mr Lutton has also asked the Police Ombudsman to investigate claims his father's killers were shielded from justice.

According to Mr Simpson the person he is planning to name was allegedly involved in other serious offences but was later recruited as an informer.

He added that he is not planning to name the republican before the autumn when the Assembly will have returned after the summer break.

"It will be October time before we get to the nitty-gritty of it. At this moment in time we are keeping our powder dry."