Members of the Republican Sinn Fein party voted overwhelmingly on Sunday to begin co-operating with the Northern Ireland police.
The vote, by hundreds of party members at aconference in Dublin, formally abandons Sinn Fein's decades-old hostility to legal law and order in the British territory.
The result was confirmed by a sea of raised hands, but no formally recorded vote.
British Prime Minister Tony Blair said the result increases the chances of reviving a Catholic-Protestant administration, the long-elusive goal of the 1998 Good Friday peace pact, by Britain's deadline of March 26.
"We will determine whether we have a basis for the future of Northern Ireland that allows us both to have power-sharing between Unionists and Republicans … and have a situation where everybody in Northern Ireland supports and supports properly the police, the courts and the criminal justice system."
The vote is considered a triumph for Sinn Fein president Gerry Adams.
The former Irish Republican Army commander has spent 24 years edging his party away from violence and toward compromise.
Sinn Fein is the political wing of the Irish Republican Army, which often targeted police during itsarmed campaign against British rule.
Adams called Sunday's decision "historic" andpraised members for a move he said would give the party greater political strength.
"Today you have created the potential to change the political landscape on this island forever," Adams told the conference.