The Northern Ireland Police Ombudsman's office has said the British government's decision not to grant extra funding to investigate three controversial killings will lead to considerable delay in the cases.
Detectives working for the Ombudsman, Mrs Nuala O'Loan, are investigating events surrounding the cases which include the high-profile murder of Catholic RUC sergeant Joe Campbell, in Cushendall, Co Antrim, 26 years ago.
The Campbell family claim other police officers, up to a senior level, were involved in the killing because Sgt Campbell had uncovered evidence of collusion among police, the intelligence services and loyalist paramilitaries.
The office is also investigating the death 12 years ago of a teenager from Portadown, Co Armagh, who was shot in an RUC man's car.
Ms Alice McLoughlin (16) died from a shot discharged from a pistol owned by the officer, who remains in the police.
The coroner at the inquest found her wounds consistent with self-infliction.
The McLoughlins are insisting their daughter could not have been responsible for her own death.
The third case cited is the 1997 killing of a Protestant, Mr Raymond McCord (22).
His family are claiming the murder was sanctioned by a UVF member, who was also an RUC informer.
Last week, the Northern Ireland Office denied Mrs O'Loan's request for additional funding to investigate the cases.
An NIO spokesman said the request had been "carefully considered" but was rejected "in light of financial pressures on the NIO's budget".
A spokesman for Mrs O'Loan's office expressed disappointment with the decision.
"We are not dropping any of the cases but there will now be considerable delay in our investigations," he said.
"Work has already been done on these cases but there remains a considerable amount to be done and we need more resources if we are to do it properly."