The public inquiry into the murder of Rosemary Nelson has been postponed until September 2007 at the earliest.
Rosemary Nelson died after a booby-trap bomb attack on her car
The inquiry has said it will not be possible to complete outstanding work to allow the full hearing to begin as planned next January.
It said that because of the amount of work still to be done, it is not able to set a definite new date for the start of the hearings.
Solicitor Mrs Nelson, 40, was killed by a car bomb in March 1999.
Loyalists planted a booby-trap bomb underneath the mother-of-three's car outside her Lurgan home.
The inquiry is one of four being held in Northern Ireland into claims of collusion.
The others are into the murders of Robert Hamill, Billy Wright, and Pat Finucane.
A fifth inquiry, into the murders of two senior RUC men, is being held in the Irish Republic.
Retired English High Court Judge Sir Michael Morland had been due to start hearing evidence in the Rosemary Nelson inquiry on 16 January 2007
A statement from the inquiry panel said: "The inquiry recognises that this news will disappoint some of those who are concerned with its work.
"It has reached its decision only after carefully considering the progress made to date in all areas of its work and the adverse consequences of deciding to maintain the present start date."
SDLP assembly member Dolores Kelly said the Nelson family "must be given a fuller explanation in private".
"It is always a bad idea to delay inquiries of this kind, because justice delayed can indeed be justice denied," she said.
Sinn Fein assembly member John O'Dowd said the murder "remains an emotive issue for nationalists in this area" and people would be "suspicious and concerned" about the delay.
"Many people remain deeply sceptical about the willingness of the British government and its securocrats to fully co-operate in exposing the truth about collusion in this society, and these sorts of delays will fuel that scepticism," he said.