The prime minister has insisted that the police and MI5 will operate as distinct bodies in Northern Ireland.
In a written statement to the House of Commons, Tony Blair said police and the security services would be "completely distinct and entirely separate".
MI5 are due to take over responsibility for national security issues in Northern Ireland later this year.
Sinn Fein welcomed the statement, but the SDLP said it did not resolve concerns about oversight of security.
Mr Blair stressed MI5 would not have any role in civic policing - a move aimed at reassuring republicans.
"When anybody has cause for concern or complaint about national security intelligence-gathering, unlike now, there will be nobody credible to turn to"
He said he hoped his statement would reassure Sinn Fein and help republicans move to support the police.
Questions about the role and accountability of MI5 have dominated the debate about policing in Northern Ireland in recent weeks.
Mr Blair also announced that police officers would not be seconded to work under the control of MI5.
But he said there would be liaison between them in response to the threat of international terrorism, as directed by the chief constable.
As expected, the prime minister also announced that Lord Carlile, the government's independent monitor of anti-terrorism legislation, will review annually the operation of the arrangements for handling national security related matters in Northern Ireland.
'Malign and corruptive'
SDLP leader Mark Durkan warned the measures would limit the ability of the Police Ombudsman to investigate national security issues.
"When anybody has cause for concern or complaint about national security intelligence-gathering, unlike now, there will be nobody credible to turn to," he said.
Sinn Fein policing spokesman Gerry Kelly said local policing would be protected from the "malign and corruptive control of MI5".
"The whole issue of MI5, and these security services are also in the south of Ireland, is that if they act illegally then we have a PSNI which is not signed up to MI5 and which will hold them to account."
"If (MI5) act illegally then we have a PSNI which is not signed up to MI5 and which will hold them to account"
DUP MP Nigel Dodds said Sinn Fein's support for policing and justice would have to be "tested by their actions".
"Words alone are not enough. Sinn Fein's support for the police, the courts and the rule of law has to be tested by their actions over a credible period of time because we have learned to our cost in the past that Sinn Fein's words are meaningless when it comes to translation into action," he said.
The UUP's Fred Cobain said Mr Blair's statement "may have offered much delight for Sinn Fein," but it had created "a confused mess for the rest of us to deal with".
"I fear that, in his rush to appease Sinn Fein, Mr Blair has lost his way completely by raising further question marks over the policing and justice ministry," he said.
"Sinn Fein have lost the battle on policing, but due to bad negotiating tactics by the DUP, we're seeing the government continuing to pander to republicans."
MI5 is building a £20m headquarters in Palace Barracks outside Belfast.
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