The assassination of a Sinn Fein official who had admitted spying for the British will make a new power-sharing deal in Northern Ireland more difficult, Irish Prime Minister Bertie Ahern told lawmakers Wednesday.

Despite this, Ahern said he and British Prime Minister Tony Blair would reveal a long-awaited new plan Thursday in Armagh, Northern Ireland, that aims to forge a Catholic-Protestant administration led by leaders of Sinn Fein, the IRA-linked party, and their bitter Protestant rivals from the Democratic Unionist Party.

"We have made it clear that the two governments will not be deterred from efforts to bring politics center stage in Northern Ireland," Ahern said. "So Tony Blair and I will travel to Armagh tomorrow and get on with it."

Ahern said Tuesday's killing of Denis Donaldson in northwest Ireland had heightened Protestant opposition to sharing power with Sinn Fein. He said the killing "certainly makes it more difficult."

Donaldson, 55, had been living alone at his family's vacation home since December, when he went on national Irish television to confess he had been a British agent inside Sinn Fein for decades. The IRA traditionally killed anybody suspected of spying for the British, but the outlawed group last year declared it had renounced violence and disarmed — and on Tuesday denied killing Donaldson.

The police officer leading the investigation, Chief Superintendent Terry McGinn, said Donaldson "died from shotgun blasts to the body" and that the front door had been broken down.

McGinn said detectives were combing a two-mile perimeter around the property and had already found objects of potential evidence, which she declined to specify.

Earlier, Ahern told lawmakers that Donaldson appeared to have been hit twice by shotgun blasts at close range. The attack, he said, severed Donaldson's right hand.

He said Donaldson's betrayal of comrades in Sinn Fein and the IRA meant he had many enemies. "But whoever was responsible for this evil deed was certainly no friend of the peace process," he said.

In London, Blair called the killing "a serious, appalling, barbaric act." He said it would have "serious implications" if police determined that IRA members were responsible, and emphasized that Sinn Fein "has to be clean of all this." But he said delaying Thursday's announcement would only reward the killers.

"The timing of this (killing) does suggest that whoever did this wants to derail the peace process. Our response should be to deny them what they want," Blair said.

Blair added that dissident IRA hard-liners opposed to Sinn Fein's political direction might have killed Donaldson specifically to sabotage the new diplomatic effort. "Sometimes these things can be perpetrated by people in disagreement with their leadership," he said.

Sinn Fein deputy leader Martin McGuinness dismissed the idea that the IRA had sanctioned the hit on Donaldson. He argued that the IRA had taken difficult, landmark decisions last year to disarm and renounce "armed struggle" in hopes of promoting power-sharing.

"It's hardly likely the IRA would take these enormous initiatives and then move on to do something stupid like attack Denis Donaldson," McGuinness said.