Prime Minister Tony Blair is set to detail MI5's future role in Northern Ireland in a new move to break the deadlock over republican support for policing.
Mr Blair is to make a statement in the House of Commons tomorrow outlining the type of work the agency will carry out when it assumes responsibility for national security at the end of this year.
MI5 is building a £20 million (€30 million) headquarters in Palace Barracks outside Belfast.
Sinn Féin has insisted there must be a complete separation of MI5 from civic policing. Police Service of Northern Ireland chief constable Sir Hugh Orde has backed MI5 assuming control of national security issues.
But the SDLP and Al Hutchinson, the oversight commissioner for police reforms in Northern Ireland, have expressed concerns about the implications of MI5 taking on the role for police accountability structures.
As Northern Ireland Secretary Peter Hain prepared to brief an Assembly group on his proposals for a future Justice Ministry at Stormont, Minister for Foreign Affairs Dermot Ahern confirmed the political process was reaching a critical phase.
"I believe all parties should take time to reflect on how far we have come and the opportunity we risk losing, potentially for years," he said. "I believe the public will not easily forgive political leaders for failing at this critical juncture. We need less public statements and what we need now is a time of quiet reflection and hard work behind the scenes.
"The Irish government will spare no efforts to secure a deal."
Mr Ahern said: "Obviously the British government have their own security service mechanisms and architecture, but in the context of what we are trying to do in Northern Ireland I hope we know some details already that Tony Blair's statement will be helpful to the situation."
Sinn Féin policing spokesman Gerry Kelly said his party was awaiting with interest the Prime Minister's statement which he hoped would undo the proposals in the St Andrews Agreement.
"The proposals which the SDLP claimed to have negotiated at St. Andrews were completely unacceptable," the North Belfast Assembly member said. "They would have embedded MI5 within the PSNI. This would have generated the potential to create once again a force within a force.
"There can be no integration of MI5 and the PSNI. All PSNI members must be fully answerable to the Patten accountability mechanisms, hard won in negotiations."