The DUP is preparing to use parliamentary privilege to claim that a senior Sinn Féin figure conspired in the murder of a former RUC officer and was later recruited by police as an informer.
Whether the allegation is true or false, it will send shock waves through the Stormont power-sharing executive. Relations between the DUP and Sinn Féin have been excellent since the two parties entered government together last May.
David Simpson, the DUP MP for Upper Bann, is considering naming the Sinn Féiner in the House of Commons. "The person concerned was alleged to have been involved in other serious offences but was later recruited as an informer," he said yesterday. "The information that this individual provided was used to reduce the capabilities of the IRA."
The DUP have used parliamentary privilege on a number of occasions in the past to accuse people of being republican terrorists, and not always correctly.
The Sinn Féin politician now under suspicion is a well-known figure whose position is arguably as sensitive as that of Denis Donaldson, who was exposed as a police agent two years ago. Donaldson was later shot dead in Donegal.
The murder victim in the case referred to by the DUP is Frederick "Eric" Lutton, 40, a former part-time member of the RUC reserve who was shot dead on May 1 1979 in Tyrone. Lutton worked as a caretaker for the National Trust and was killed as he got out of his car to lock the gates of their premises in Moy.
The father-of-two was a cousin of Simpson, and the MP has raised the case within the party. Several other senior figures in the DUP now know the details.
This weekend Lutton's family refused to comment on the case, which they hope will be re-opened by the PSNI's historical inquiries Team (HET). In an interview with the Belfast News Letter last year, his son Nigel said two masked IRA men had lain in wait behind pillars alongside the gates of Argory house in Moy.
"He opened the car door to get out and the two Provos came out from either side. They had Belgian FN assault rifles, really heavy-duty stuff, and they caught him between the door and the car. They just emptied high-velocity rounds into him."
He said that the family was threatened afterwards and told to leave the Annaghmore area where they still live. He was approached to join the UVF in order to get revenge but refused. Two men were arrested on suspicion of being the gunmen, but later released without charge.
A number of police officers have told DUP researchers that the Sinn Féin figure helped monitor Lutton's movements, a process known as targeting, as well as removing the weapons after the attack. The former police officers also say the politician was recruited as a police agent about two years later.
DUP sources say these officers are prepared to give evidence to the HET, and that several senior DUP figures are prepared to push the issue whatever the political fall-out. "We are confident that this person targeted Eric Lutton for murder, disposed of the weapons afterwards and was later recruited as a police agent partly because of being caught in a sexually compromising position and partly for other reasons," a party source said.
Simpson has received assurances from Sir Hugh Orde, the PSNI chief constable, and Paul Goggins, Northern Ireland security minister, that prominent people will not be protected from prosecution for serious crimes because of favours done for the police or intelligence services.
Earlier this year at the Northern Ireland committee at Westminster, Simpson demanded that there be no repeat of the Donaldson affair, "in which his role as an informant clearly played a part in the suppression of a criminal prosecution. If the team exposes a senior politician, the law should be allowed to run its course".
Speaking at a Sinn Féin rally in Belfast last weekend, Gerry Adams agreed that British agents who worked within Sinn Féin or the IRA should be investigated.
He said: "The British recruited, blackmailed, tricked, intimidated and bribed individual republicans into working for them and I think it would be only right to have this dimension of British strategy investigated also."