Sinn Féin has voted in favour of supporting policing at the party's Ard-Fheis in Dublin.
The motion was carried following a show of cards from the 900 delegates.
The debate, which went on since early this morning, saw very little opposition to the motion supporting policing.
It was passed following a series of amendments, which Sinn Féin said would enhance the proposal.
Following the vote, the party president, Gerry Adams, said the decision created the opportunity to significantly advance the party's primary objective of an united Ireland.
He added: 'The decision we have reached is truly historic...One of the most important in the recent history of our country.'
Decision receives broad welcome
The Sinn Féin decision in favour of supporting policing in Northern Ireland for the first time ever has been welcomed in Dublin, London and Belfast.
The Taoiseach said: 'This is a landmark decision and opens the way for inclusive support for policing throughout Northern Ireland.'
The move has been seen as essential to help pursuade the Democratic Unionist Party to keep its commitment to power-sharing in Northern Ireland.
Ian Paisley's DUP said the time for true and visible Sinn Féin support for policing had arrived.
However, Mr Paisley said anything less than full implementation of Sinn Féin's committments would render the Ard-Fheis vote valueless.
He added: 'Only with real delivery can the way be cleared for a full return to democracy and a facing up to the everyday needs and requirements of the people of Northern Ireland.'
PSNI Chief Constable Hugh Orde said everybody was entitled to a policing service.
He said: 'Today's decision is welcome. 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.'
Northern Secretary Peter Hain said the overwhelming support by republicans for policing and the courts put Northern Ireland in a place it had never been before.
The Ulster Unionist leader Reg Empey described the Sinn Féin vote as a massive step change in the republican psyche.
He said: 'It is an admission that the violenct cause has been abandoned and that Sinn Féin are prepared to support the force of law and order.'
The SDLP leader Mark Durkan welcomed the Sinn Féin vote, saying: 'Today's decision raises hopes that Sinn Féin will now sign on for policing and that this would be matched by the DUP committing to restoration and powersharing.'