Northern Ireland's lawmakers have approved a landmark deal transferring key powers from London to Belfast, in what British Prime Minister Gordon Brown hailed as the "final end" to decades of conflict.

The vote in the Northern Ireland Assembly on Tuesday was passed by 88 votes to 17 after a stormy debate and, in the hours leading up to it, a rare intervention from former US president George W. Bush.

Leaders of the two main power-sharing parties voted in favour of switching control over policing and justice powers from London to Belfast, but failed to persuade the Ulster Unionist Party (UUP) to join them.

Despite the opposition, Brown hailed the deal as the final stage in creating a devolved settlement in Northern Ireland, which was wracked by three decades of violence before a first peace agreement in 1998.

"Today the politics of progress have finally replaced the politics of division in Northern Ireland," said Brown.

"The completion of devolution... is the final end to decades of strife. It sends the most powerful message to those who would return to violence: that democracy and tolerance will prevail."

US Secretary of State Hillary Clinton praised the agreement as an "important step for the peaceful and prosperous future for all of the people of Northern Ireland for generations to come."

She said she planned to host talks in Washington next week with the province's leaders about "the way forward".

Under the deal, policing and justice powers -- a highly sensitive issue due to Northern Ireland's bloody sectarian history -- are expected to transfer from London to Belfast on April 12.

The agreement was carved out last month after days of negotiations between the Democratic Unionist Party (DUP) and Sinn Fein, the one-time foes who now share power in a devolved government.

Irish Prime Minister Brian Cowen called the vote "historic", adding: "This is a good day for the people of Northern Ireland and for everyone on this island."

"This is a further vote of confidence in Northern Ireland’s future and further proof that there is no going back to the days of hatred and conflict," he said.

On the eve of the vote, former US president Bush had called the leader of Britain's main opposition Conservatives, David Cameron, in the hope of persuading him to talk his UUP allies out of opposing the deal.

Bush spokesman David Sherzer said: "President Bush and his administration played an active diplomatic role in promoting lasting peace and prosperity in Northern Ireland.

"He views the reconciliation in Northern Ireland as an example for other societies around the world."

Although a UUP "no" vote does not wreck the deal, there are fears the accord could prove unsustainable without all-party support.

The Conservatives are slightly leading Brown's ruling Labour party in opinion polls ahead of a general election likely on May 6. The Tories will field joint candidates with the UUP in Northern Ireland.

After Tuesday's vote, UUP leader Reg Empey defended his party's decision to oppose the deal, saying it had refused "to bow to the blackmail and bullying to which we have been subjected in recent weeks".

The UUP, like the DUP, is Protestant and favours Northern Ireland remaining part of Britain. Sinn Fein is Catholic and wants Northern Ireland to become part of a united Ireland.

Northern Ireland's three decades of violence, known as The Troubles, in which more than 3,500 people died, were largely ended by the 1998 Good Friday Agreement.

The main paramilitary groups including the Irish Republican Army (IRA) have laid down their arms, but sporadic violence still plagues the province, including the killing of two British soldiers and a policeman last year.