Operation Ballast has investigated a number of UVF murders
The SDLP has criticised a decision to transfer an investigation into a series of murders away from the Historical Enquiries Team (HET).
Detectives from the Serious Crime Branch of the PSNI are to take over the running of Operation Ballast.
It has investigated a series of murders and other crimes by the the UVF in north Belfast amid allegations of collusion with Special Branch.
SDLP policing spokesman Alex Attwood said he had concerns about the plan.
It is understood Operation Ballast will now investigate at least another 20 murders.
The PSNI said the investigation was "large, complex and active" and that the Serious Crime Branch was "the most appropriate mechanism" to continue with it.
It did not offer any further explanation about why the HET was no longer appropriate.
Operation Ballast has already re-examined at least 10 deaths blamed on the UVF gang in the Mount Vernon estate in north Belfast.
The original probe was set up amid claims that the security forces colluded with UVF members over the killings.
Former Northern Ireland Police Ombudsman Nuala O'Loan, claimed paramilitary killers were protected from prosecution because they were police agents.
She examined the allegations after a complaint from Belfast man Raymond McCord, whose son, Raymond McCord Junior, was murdered by the UVF in 1997.
Mr Attwood said he had a number of concerns and the move required "the greatest scrutiny".
He said that families and victims had "a level of confidence" in investigations carried out by the HET and this may not cross over to a PSNI-led investigation.
"The SDLP will listen to why the PSNI believe the Ballast investigation will be more complete and more successful if conducted by the PSNI," Mr Attwood said.
"However, those arguments must be balanced carefully and fully against the success of the Historical Enquiries Team to date and in developing evidence to the point of prosecution and the need to keep confidence of families, victims and the wider community.
"Only when all of these factors have been addressed should a decision be made about the work of the Historical Enquiries Team, Operation Ballast and the investigations."