Sinn Fein members have voted to support policing in Northern Ireland for the first time in the party's history.
About 900 party members voted on the motion at a special party conference (ard fheis) in Dublin which was attended by more than 2,000 people.
Sinn Fein support for policing and DUP commitment to power-sharing are seen as essential to restoring NI devolution.
A six hour debate was cut short as the leadership forced a vote which was carried with 90% support.
The decision gives Sinn Fein's ruling executive the authority to declare its support for the PSNI and the criminal justice system when devolution is restored and policing and justice powers are transferred to the Northern Ireland Assembly.
Speaking after the vote, Sinn Fein President Gerry Adams said the decision was truly historic.
"Today you have created the potential to change the political landscape on this island forever," he said.
"You have created the opportunity to significantly advance our struggle and you have seized the opportunity to further our primary objective of united Ireland through the building of greater political strength."
Mr Adams also said that republicanism and unionism had reached an historic compromise.
"If the promise and hope of the peace process is to deliver peace and prosperity, that means beginning a real dialogue, an anti-sectarian dialogue, a dialogue which will move us to a real future," he added.
A spokesman for Tony Blair said the prime minister welcomed the "historic decision and recognised the leadership it has taken to get to this point".
Irish Taoiseach Bertie Ahern said the ballot was a "landmark decision" which opened the way to Northern Ireland power-sharing.
He said: "It is vital that we continue to maintain the momentum from the St Andrews agreement and the timetable set out in that agreement."
Sir Hugh Orde said he welcomed the Sinn Fein vote
Northern Ireland Secretary Peter Hain described the vote as a breakthrough.
"What had always been a massive impediment to stable and lasting government (in Northern Ireland) has been removed," he said.
PSNI Chief Constable Sir Hugh Orde also welcomed the move.
"Our view has always been that policing is a public service which every member of the community should be able to access on an equal and equitable basis," he said.
"I have always said that no ideology or individual should stand between the public and that service and that the community is entitled to have their public representatives hold this police service to account."
Professor Sir Desmond Rea, chairman of the Policing Board, said he was now looking forward to Sinn Fein joining the body.
"Full political and community support for policing will be for the benefit of the whole community," he added.
DUP MP for East Antrim Sammy Wilson said he accepted Sinn Fein had taken a step forward.
He said: "The ultimate test of this, because there is no trust in Sinn Fein, is will they deliver on supporting policing before they get into government?
"They cannot get into government and not support the police."
Ulster Unionist leader Sir Reg Empey said the move was "a massive step change in the republican psyche".
"It is an admission that the violent 'cause' has been abandoned and that Sinn Fein are prepared to support the forces of law and order in this part of the United Kingdom," he added.
SDLP leader Mark Durkan said: "They now need to sign with no ifs or buts. As Gerry Adams now accepts, nationalist areas need policing."
Mr Blair and Mr Ahern have identified Sinn Fein support for the Police Service of Northern Ireland (PSNI) as being crucial to persuading the DUP to share power in a devolved government with Sinn Fein by 26 March.
If an election does not occur, Stormont will be dissolved indefinitely.
The transitional assembly at Stormont will dissolve on 30 January in anticipation of an election on 7 March.