Tony Blair has promised that MI5 will have no role in policing in Northern Ireland, in a bid to reassure republicans wary of new justice arrangements in the province.
In a statement to parliament this morning, the prime minister said: "The Police Service of Northern Ireland (PSNI) and the security service will be completely distinct and entirely separate bodies."
MI5 will take over control of national security in Northern Ireland from April, but Sinn Fein is deeply wary of the organisation because of allegations that it colluded with the Royal Ulster Constabulary (RUC) during the Troubles.
Discussions on restoring power-sharing to the province are at a crucial stage, as Sinn Fein decides whether to drop its historic opposition to the PSNI - a key requirement to restoring devolution under the St Andrews agreement.
If all goes well, power could be restored to the Stormont assembly by the end of March and full responsibility for policing and justice in the province would be transferred from Westminster within a year.
But yesterday Sinn Fein policing spokesman Gerry Kelly said the "complete separation" of the PSNI and MI5 was "a priority" for his party, warning: "There can be no integration of MI5 and the PSNI."
In his statement, Mr Blair said: "Policing is the responsibility solely of the PSNI. The security service will have no role whatsoever in civic policing."
The prime minister said the decision to give MI5 control over national security arrangements would bring Northern Ireland in line with the rest of the European Union, to provide a "consistent and coordinated response" to the threat of international terrorism.
He continued: "All necessary interaction between the security service and the PSNI, for example in response to the threat of international terrorism, will, as directed by the chief constable, be by way of liaison.
"No police officers will be seconded to or under the control of the security service."
Mr Blair said that under the new policing arrangements, the existing accountability arrangements would remain. This means the chief constable is accountable to the policing board, which in turn is made up of a representative mix of political parties.
The ombudsman has the power to hold all police officers to account, and in addition, Mr Blair said today that the government's reviewer of anti-terror legislation, Lord Carlile, would conduct an annual review of the national security arrangements in the province.