Party leader Sir Reg Empey said the Hillsborough deal was not acceptable in its current form after the party held all-day talks in Templepatrick, Co Antrim.
Read Ken Reid's blog: Tuesday calling
"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.
Policing and justice powers will be devolved on 12 April, if a resolution jointly tabled by the First and Deputy First Ministers gets cross-community support in an Assembly vote on 9 March.
Sir Reg said he 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."
The party will take a final decision on Monday night, he said.
Sir Reg repeatedly complained that he had been kept in the dark during the marathon talks between the DUP and Sinn Fein.
Last month, he said his party would not sign up to the deal unless a "consensus" was found to solve the education debate.
He has also been highly critical of what he calls the "dysfunctional nature of the executive".
The UUP was the only party to boycott the unveiling of the Hillsborough Agreement last month.
'Mistake'
Speaking at the Sinn Fein Ard Fheis in Dublin, the Deputy First Minister Martin McGuinness said the decision showed a dismal failure of leadership.
"Let me say to the UUP it is impossible to sit round the power-sharing table by day and court rejectionist transfers by night", Mr McGuinness said.
Earlier on Friday, First Minister Peter Robinson warned it would be a "massive mistake" for the UUP not to back the devolution deal.
Mr Robinson was speaking at a meeting of the business community in Templepatrick ahead of next week's crucial vote on the Hillsborough agreement.
"The people of Northern Ireland are very clear that they want to move forward, they want to make progress. They want their politicians to get down to the business of dealing with the economy, dealing with education and health. Those are the matters that are important to them," the DUP leader said.
"So let's finish this jigsaw, let's ensure we have devolution through, let's move forward," he added.
"It would be a massive mistake for any political party that does not give support to the assembly and the executive moving forward."
Northern Ireland Secretary Shaun Woodward said about the 9 March vote: "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."
He said it would be unthinkable to falter at this stage.
"And it would be hard to forgive anyone who put all that has been so hard won at risk," he added.
"The structures must be developed not destroyed."
Earlier this week, Mr Woodward warned extra funding worth £800m could be lost if politicians failed to back the transfer of powers.
Sinn Fein and the DUP have enough MLAs in the Assembly to secure the required cross-community vote.
But last weekend, the DUP deputy leader Nigel Dodds said devolution was unlikely to go ahead without the backing of the UUP.
The DUP's executive will meet on Monday to assess whether there is enough confidence within the community to go ahead with the deal.