Democratic Unionist Party leader, the Rev Ian Paisley said tonight that the murder of Denis Donaldson has "put a dark cloud" over the joint proposals due to be announced by Mr Blair and Mr Ahern on Thursday.
"If this man has been murdered because of his connection with IRA/Sinn Fein and because of the past happenings, then it strikes a blow at what the two governments are trying to do - to say that the IRA has forsaken these ways and they are seeking peace," he said.
The veteran politician expressed scepticism at the IRA denial of responsibility and said the killing could have implications for the joint proposals due to be announced by Mr Blair and Mr Ahern on Thursday.
"There are serious talks that are going to take place and I would say that this has put a dark cloud over those talks," Mr Paisley said.
Mr Paisley added that he had been told that Donaldson's hand had been cut off. "I have heard his hand was chopped off in this murder".
He said the murder was very strange, others who had been discovered to have been spying "were just wiped off the face of the earth".
He added: " If what I have heard is true that they cut his hand off, that would show that they were saying 'here is a hand that signed away, what we would say, his obligation to IRA/Sinn Fein and we will deal with him' - it looks like that sort of murder."
Party colleague Nigel Dodds, the MP for North Belfast, said the rush by Gerry Adams to absolve the IRA and Sinn Fein was a sign of great nervousness.
"Unionists will not be fooled into accepting the denials of Gerry Adams, a man who would still have us believe that the IRA had nothing to do with the Northern Bank robbery and denies himself ever having been a member of the IRA," he said.