The Ulster Unionists said last night that they would not be supporting the landmark deal to devolve policing powers to Northern Ireland at a crucial Assembly vote next week.
Party leader Sir Reg Empey said the Hillsborough Agreement on law and order transfer and parading was not acceptable in its current form.
While Stormont's two main parties — Sinn Fein and the Democratic Unionists — have the electoral strength to push through the accord when it is put before the Assembly on Tuesday, a “no” vote from the UUP will deprive them of the unanimous support they crave.
The party's decision is potentially problematic for the Conservative Party.
David Cameron is a supporter of the Hillsborough deal and he now faces the prospect of going to the polls aligned to a party which opposes it.
Sir Reg announced his party's position after holding all-day talks with Assembly colleagues in Templepatrick, Co Antrim.
The East Belfast MLA said the Stormont Executive needed to demonstrate an ability to address other outstanding issues facing it — such as the uncertainty over education reforms — before it could be trusted with security powers.
“It remains our view that the current Executive must be capable of exercising its existing powers before such an important issue as policing and justice is devolved,” he said.
He and his colleagues have now sent a series of proposals to the other parties in the Assembly designed to tackle the matters of concern to his party.
Sir Reg said he would examine any responses to that document over the next 72 hours and did not totally rule out a change of position ahead of Tuesday.
“The Ulster Unionist Party remains committed to devolution and to providing strong, stable and effective government for all the people of Northern Ireland,” he added.
“Our party executive will meet on Monday to hear a report from the leader and take a final decision.”
Meanwhile, speaking at Edinburgh University last night, Secretary of State Shaun Woodward said that the cross-community vote next Tuesday is nothing less than a vote for the future of Northern Ireland.
He said: “By voting to complete devolution they will be doing so much more than voting for the transfer of policing and justice powers from Westminster to Stormont, important as that is. They will be voting for the hopes and aspirations of future generations who do not want to relive the past.”