The security service, MI5, is moving to a new base in County Down, the BBC has learned.
Chief Constable Sir Hugh Orde called the plan "a healthy split"
The move to Palace Barracks, Holywood, is in preparation for an expanded role in NI intelligence gathering.
MI5 is due to take over the lead role in intelligence involving national security by the end of 2007.
Until now, the PSNI Special Branch has had overall responsibility. Chief Constable Sir Hugh Orde called the plan "a healthy split".
In future, while police handlers will continue to work with individual agents they will, in some cases, report back to MI5.
During the Troubles, MI5 officers were based at Stormont Castle - their shift of premises coincides with a shift of role.
BBC NI political editor Mark Devenport said: "From the end of 2007, they will take the lead on intelligence involving national security.
"That means MI5 will take a strategic role in countering republicans thought to represent a threat to the state - the police will continue to be in charge of agents within loyalism and intelligence related to ordinary crime."
MI5 officers were based at Stormont Castle during the Troubles
Chief Constable Sir Hugh Orde said there would be no huge influx of MI5 personnel as his officers would continue to handle agents on the ground.
"National security responsibility in NI does need to shift," he said.
"It allows me to get on with my part of the business, working closely with, and not distant from MI5 who will also be here."
The SDLP has raised concerns about the shift, arguing that MI5 will not be accountable to the Policing Board or the Police Ombudsman.
However, the DUP said the Ombudsman was negotiating understandings with MI5 which would allow her to continue to deal with any complaints.
Gerry Kelly, Sinn Fein, said that increasing MI5's role was "unacceptable".
"The role of the securocrats within both the Special Branch and MI5 needs to be reduced and ended, not supported and expanded," he said.
Mr Kelly said his party had raised the matter with both governments and would do so again.
Government sources said the change in responsibility was "not negotiable".
MI5 works from its headquarters at Thames House in London.
Its current chief, Dame Eliza Manningham Buller, is a former head of the service's Irish counter terrorism desk.