Gordon Brown tonight hailed as “the final end” to the Irish peace process a decision by the Northern Ireland Assembly to devolve policing and justice powers from Westminster, as a political row continued over David Cameron’s relationship with the Ulster Unionist Party.

Today's vote on the Hillsborough Agreement brokered between the Democratic Unionist Party (DUP) and Sinn Fein ends recent instability in the province’s power-sharing institutions, caused by a lengthy standoff between the two parties and exacerbated by a sexual and financial scandal involving Peter Robinson, the DUP leader and First Minister.

But the refusal by the Ulster Unionists to back the deal because of their exclusion from recent negotiations swung the spotlight on Mr Cameron. The Conservatives were unable to persuade the UUP, with whom they have forged a political partnership, to change their minds and give their support to the policing and justice package.

Last week former US President George Bush called Mr Cameron in a bid to get him to persuade Sir Reg Empey, the Ulster Unionist leader, to endorse the Hillsborough agreement.

Mr Brown, in a statement welcoming the vote, pointedly said: ”The courage and leadership of the parties who voted to complete devolution at Stormont will be noted around the world.”

Ulster Unionist sources told The Times that the “cack-handed” intervention of Shaun Woodward, the Northern Ireland Secretary, only hardened their resolve. Mr Woodward made a highly emotive appeal in which he linked the murder of a police officer a year ago to the vote. “We had planned to abstain from the vote until Woodward’s attack on us,” said a source.

The passing of the Assembly motion, which only UUP members voted against, opens the way for the creation of a Department of Justice after the powers are devolved from London to Belfast by April 12.

Mr Robinson accused the UUP of seeking political advantage. “Throughout history there are times of challenge and defining moments,” he said. “This is such a time. This is such a moment.

“Leadership is not about what’s easiest, or what best suits our party interests, it is about doing what is right for our people.” But the UUP claimed the power-sharing executive should sort out other outstanding matters such as the longstanding row over post-primary school transfer arrangements before it can take on law and order functions.

Sir Reg Empey said: “I am immensely proud of the sacrifices my party has made for the cause of peace. Our determination to make Stormont work for all the people of Northern Ireland - unionists, nationalists, all of us - continues.

“Our whole-hearted support for the brave men and women of the PSNI continues unabated.” He said his party refused “to bow to the blackmail and bullying to which we have been subjected in recent weeks”.

Mr Brown said in a statement: “Today the politics of progress have finally replaced the politics of division in Northern Ireland.

“The completion of devolution, supported by all sections of the community in Northern Ireland, 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."