The transfer of responsibility for matters of national security from the police to MI5 is a cause for concern, a policing watchdog has said.
New MI5 headquarters are being built in Holywood
NI Oversight Commissioner Al Hutchinson said the move had "profound potential implications for the police service".
In his 16th report, he said the transfer must not stop police from investigating organised crime.
MI5 is spending more than £20m to build new headquarters at Palace Barracks in Holywood, holding up to 400 people.
The four-storey building, which also has an underground section, extends to more than 10,000 square feet.
The oversight commissioner's report also criticised the failure to build a new police training college in Northern Ireland.
However, Mr Hutchinson said that "tremendous progress" had been made in the transformation of policing in the past five years.
The Independent Commission on Policing for Northern Ireland, also known as the Patten Commission, was set up after the Good Friday Agreement in 1998 to develop a framework for a police service capable of attracting and sustaining support from the whole community.
Mr Hutchinson is an independent figure charged with overseeing the implementation of the Patten report's 175 recommendations.
In his latest report published on Tuesday, he said 124 of the recommendations had now been completed, 10 more since his last report.
Al Hutchinson oversees the implementation of the Patten report
The decision means that from next summer, MI5 will gather intelligence on terrorist groups and their activities, while the police will gather criminal intelligence.
The oversight commissioner highlights the fact that it is sometimes difficult to tell the difference between terrorist and criminal activities, and warns that the change must not prevent the police from investigating organised crime.
He said the transfer must not affect the robust and rigorous accountability of the police.
The oversight commissioner also expressed concern about the failure to provide a new police training college, five years after the government gave a commitment to do so.
The government has pledged £90m to the project, a figure which is £40m less than what is needed.
Mr Hutchinson said the reforms were irreversible and had gone so far that there was nothing to stop Sinn Fein joining the Policing Board.
He said the political judgement from Sinn Fein that the time was right to act was the only thing needed.
"Virtually everything is on a good track to improvement, and I don't know why they would hesitate in joining from a practical perspective," he said.
Policing Board chairman Professor Sir Desmond Rea said he welcomed Mr Hutchinson's comments on the police college.
"While the government has made available £90m there remains a shortfall, without which the college, as planned, simply cannot be built," he said.
PSNI Assistant Chief Constable Roy Toner, who has responsibility for the management of change within the service, said: "Creating and managing change in a large organisation like ours, while at the same time delivering a quality policing service, is not easy, but we are doing it."
Mr Hutchinson, an officer with the Royal Canadian Mounted Police for 34 years, took over as Oversight Commissioner from Tom Constantine who held the position from May 2000 until his retirement at the end of 2003.