The Northern Ireland Assembly has voted to accept a motion devolving policing and justice powers, in a historic moment for Stormont.
An unexpected intervention from former US president George Bush failed to persuade the Ulster Unionist party (UUP) to back the vote, however, leading to concerns that powersharing remains vulnerable in the province.
Despite their opposition, Gordon Brown hailed the completion of devolution as a victory for the "politics of progress" over "the politics of division".
"It sends the most powerful message to those who would return to violence: that democracy and tolerance will prevail," the prime minister said.
"The courage and leadership of the parties who voted to complete devolution at Stormont will be noted around the world."
The UUP rejected the proposal after lengthy negotiations between the powersharing Democratic Unionist party (DUP) and Sinn Fein last month.
Ex-US president Mr Bush had contacted David Cameron, whose Conservatives are fielding joint candidates with the UUP in Northern Ireland, to ask him to influence the deal.
Mr Cameron said it was "good and constructive" that Mr Bush and other Americans took an interest in Northern Ireland.
"What I said is we have played the most constructive role we possibly could as an opposition," he commented before the vote.
"We've done everything we can to encourage all unionists to back the devolution of policing and justice... but the one thing we can't do is force people to vote a particular way."
The UUP's chief whip, Fred Cobain, had attacked the "semi-detached" Northern Ireland secretary Shaun Woodward's threat of withholding the £800 million of funding for policing and justice as "nothing short of blackmail".
"If Shaun Woodward wanted the UUP's support, he should have guaranteed the UUP's place at the negotiating table," he said.
Commentators have warned that the UUP's hardline opposition could undermine the support of the more uncompromising wing of the DUP, risking the slow break-up of the current powersharing consensus.
The motion was passed by 88 votes to 17.