David Cameron’s authority was questioned last night after his Unionist allies voted against the final stages of devolution in Northern Ireland.

The Ulster Unionists were the only party that failed to back the transfer of police and justice powers to Stormont. The successful vote, which will set up a Department of Justice for Northern Ireland next month, was hailed as a landmark by London and Dublin.

Gordon Brown said it marked “the final end” to the peace process. In a joint statement, the Prime Minister and the Taoiseach, Brian Cowen, said it marked a “significant step forward”. But Mr Cameron had to explain why his attempts to bring the Ulster Unionists on board had failed.

“The one thing we cannot do is force people to vote a particular way,” he said.

The Conservatives’ relationship with the Ulster Unionists has come under the spotlight because of Mr Cameron’s decision to go into an electoral pact with the once dominant voice of Unionism.

The parties, whose historic alliance was broken in the 1970s, will share Northern Ireland candidates at the general election. The successful ones will take the Tory whip at Westminster — Commons votes that could be crucial in allowing Mr Cameron to govern. He has come under pressure from the US, including from George W. Bush, the former President, to bring his influence to bear on Sir Reg Empey, the Ulster Unionist leader.

The symbolism of all parties voting the Hillsborough agreement through the Northern Ireland Assembly was seen as a critical message to both communities.

A senior Labour source said of Mr Cameron: “Either he didn’t fully use his influence or he doesn’t have any influence.”

Yesterday’s vote ends recent instability in the province’s power-sharing institutions, caused by a stand-off between the two leading parties — the Democratic Unionist Party (DUP) and Sinn Fein — and exacerbated by a sexual and financial scandal involving Peter Robinson, the DUP leader and First Minister. Mr Brown pointedly praised “the courage and leadership” of the parties that had backed devolution.

Ulster Unionist sources told The Times that an intervention by Shaun Woodward, the Northern Ireland Secretary, hardened their resolve. He made an emotive appeal linking the murder of a policeman a year ago to the vote. “We had planned to abstain until his attack on us,” said a source.