A full public inquiry into Stormontgate is needed, the Ulster Unionist Party has said.
Police raided Sinn Fein offices at Stormont
It follows the expulsion of a Sinn Fein member from the party acquitted of involvement in an alleged IRA spy ring.
"It is now utterly ludicrous to suggest that the case is not in the public interest," the party's Michael McGimpsey said.
He said it was "hard to see how we can make any momentum in the New Year before this is properly investigated".
All charges against the party's former Stormont head of administration, Denis Donaldson, and two other men were dropped by the Crown.
On Friday, Mr Donaldson confirmed that he had been a British agent for two decades.
In a statement the DUP said a detailed explanation on the issue was needed from the government.
"It is now time for a full and detailed explanation from the government and for them to explain was this the reason for the dropping of the charges against those accused," it said.
"If not why was the decision taken by the DPP."
SDLP Deputy Leader Dr Alasdair McDonnell said Sinn Fein had to "come clean" about the issue.
"In our meeting with the Attorney General it was made clear that the Stormontgate prosecution was not dropped because of any weakness in the evidence," Mr McDonnell said.
"We suspected that the public interest argument was a way of covering up for an informer. We don't know whether Denis Donaldson was that particular informer.
"There is the distinct possibility, like so many cases in the past, that he is being used as a scapegoat to cover someone else."