Unionists in Northern Ireland have called for a public inquiry after an expelled Sinn Fein official admitted he was a British agent.
Denis Donaldson said allegations of an IRA spy ring which led to the collapse of power-sharing had been "a scam and a fiction" invented by UK intelligence.
Last week he was one of three men cleared of gathering intelligence for the IRA at Stormont.
The DUP has said Tony Blair should make a Commons statement on the subject.
DUP MEP Jim Allister said the prime minister must be prepared to give answers.
"The prime minister is the ultimate head of security. He must, therefore, know that Donaldson was an agent," he said.
"If the prosecution was abandoned because Donaldson was an agent, the prime minister knows that is the reason.
"Yet in the House of Commons he went on record to say he had no idea why this prosecution was abandoned. Well, was he being economical with the truth? I think he has some explaining to do."
Martin McGuinness of Sinn Fein said a "British spy ring" had been operating at Stormont.
"What we believe was going on was a spy ring at Stormont with the purposes of collapsing the institutions established under the terms of the Good Friday Agreement," he told the BBC's Inside Politics programme.
He said republicans would not be surprised at "yet another episode in the dirty war of the British security services".
However, former UUP leader David Trimble, who was Northern Ireland's first minister when the three men were arrested in October 2002, rejected claims the affair had been engineered by British intelligence.
"There is a spin going on here and the spin is going on because actually it's the republican movement that's in a crisis today," he told the BBC's Today programme on Saturday.
"They have discovered that a person who was in a very senior level within Sinn Fein in fact was operating as a agent for over 20 years."
UUP leader Sir Reg Empey said people had a right to know what had happened.
"The very least the prime minister and the Northern Ireland secretary can do is to hold a public inquiry," he said.
"Huge amounts of tax payers' money has been spent relocating prison officers and others and I think tax payers are entitled to know why that money was spent and we are also entitled to know what actually happened."
Mr Donaldson was expelled from Sinn Fein on Friday, and later said he had been recruited in the 1980s as a paid agent and deeply regretted his activities.
Sinn Fein leader Gerry Adams claimed Mr Donaldson had been about to be "outed" by the same "securocrats" who had set him up as a spy.
Mr Adams claimed Mr Donaldson had been approached by police officers last week and told he was about to be "outed" as an informer.
He said Mr Donaldson was not under any threat from the republican movement.
Northern Ireland's power-sharing executive collapsed in October 2002 following the arrests of three men, including Mr Donaldson, who had headed the party's administration office at Stormont.
Charges against the three were dropped last week after the prosecution offered no evidence "in the public interest".
Unionists have said Mr Donaldson's statement proved the charges against all three were dropped "in a deal with the IRA" to secure decommissioning.
However, both the Northern Ireland Office and the Police Service of Northern Ireland have denied this.
On Friday, the Northern Ireland Office said it "completely rejected any allegation that the police operation in October 2002 was for any reason other than to prevent paramilitary intelligence gathering".
SEQUENCE OF EVENTS
4 October 2002: Three men arrested following raid on Sinn Fein's Stormont office. Power-sharing executive collapses and government restores direct rule to NI a week later
8 December 2005: Charges against three men dropped "in the public interest"
16 December 2005: Sinn Fein says Denis Donaldson was a "British agent" and expels him from the party: he later says he worked as a spy since the 1980s
Government and police reject the party's claim raid was politically motivated
It said "the fact remains that a huge number of stolen documents were recovered by the police".
Police sources earlier reiterated that the "Stormontgate" affair began because "a paramilitary organisation was involved in the systematic gathering of information and targeting or individuals".
But Sinn Fein has always insisted there never was a spy ring, calling the whole business "a politically-motivated stunt to discredit republicans".
In a statement to Irish broadcaster RTE on Friday, Mr Donaldson said: "I was a British agent at the time. I was recruited in the 1980s after compromising myself during a vulnerable time in my life.
"Since then I have worked for British intelligence and the RUC/PSNI Special Branch. Over that period, I was paid money.