Sinn Fein MLA Francie Molloy, who was accused under Parliamentary privilege of involvement in the killing of a former RUC reserve officer almost 30 years ago, has accused the DUP of carrying out a "cheap political stunt" .
The prominent republican insisted there was "no basis" to allegations that he was involved in the murder of Frederick John (Eric) Lutton in 1979, or that he was a security agent who helped bring down the IRA's east Tyrone brigade.
The claims were made by Upper Bann MP David Simpson, a cousin of the murdered man, who used parliamentary privilege to level the accusations against Mr Molloy during an adjournment on policing in the House of Commons last night.
Using the legal protection given to MPs, the DUP man alleged Mr Molloy was a suspect during the early murder investigation before becoming a "less than willing informer" after police uncovered a series of sexual indiscretions involving Mr Molloy.
During a 10-minute address to parliament, Mr Simpson described the murder of his cousin, who was shot dead as he locked gates at The Argory House near Moy on May 1, 1979.
Mr Molloy said: "I refute Mr Simpson's allegations completely. They are scurrilous accusations, they are without base and I challenge him to make the allegations public without protection of Westminster and parliamentary privilege.
"Regarding his claims that I was a British agent, I know nothing at all about them. His allegations are completely untrue and at no time was I compromised.
"It is a cheap political stunt that says more about Mr Simpson that it does about me."
The 57-year-old Sinn Fein veteran, who, 24 hours before the allegations was nominated to serve on Dungannon's district policing partnership, also questioned if Mr Simpson had "any genuine concerns" for the victim's family.
Attempts were made to contact Mr Lutton's son Nigel, but he refused to make any comment.
Mr Lutton (40) had worked as a caretaker for the National Trust for 15 years and was a part-time member of the RUC reserve, although he had resigned several months before the attack.
The IRA later claimed responsibility for his murder but no one has ever been charged.
Following the allegations, Justice Minister Paul Goggins said: "Mr Lutton was one of the 3,368 innocent people who lost their lives during the Troubles needlessly and often in a very violent way. All of those deaths have been referred to the historical enquiries team and each will be looked at."