The DUP has been challenged to publish written assurances they claim were given by Tony Blair over the ministerial pledge of office.
Sir Reg Empey wants the DUP to publish the allege letter
Ulster Unionist leader Sir Reg Empey said he did not believe such a letter from the prime minister existed.
The DUP wants a pledge of support for policing in place before Ian Paisley and SF's Martin McGuinness can become shadow first and deputy first minister.
Sir Reg said the DUP might only have a written minute of talks with the PM.
"I call on Ian Paisley to publish these written assurances," he said.
"We don't believe he has a letter from the prime minister.
"What he may have is a minute of a meeting with him, which might say the prime minister has promised to amend the pledge of office so that there will be support for the police written into it."
Mr Paisley and Mr McGuinness are due to take up their shadow roles on 24 November.
The DUP's Ian Paisley Jnr has claimed the deputy first minister designated on that date would have to "swear and pledge allegiance to serve the interests of the Ulster people but also to support the only police service and Royal Courts of Justice".
On Tuesday, the DUP leader claimed he had promises from the government in writing and if those promises were not kept the "writings" would be taken out and pushed down the government's throats in public.
Downing Street has refused to comment on the matter.
The meeting between the party leaders was postponed
Meanwhile, Sinn Fein's Gerry Kelly said the party was still debating whether to back the PSNI but was not ready to vote on it.
Mr Kelly was speaking after a meeting to be attended by both Ian Paisley and Gerry Adams on Tuesday was postponed.
Northern Ireland Secretary Peter Hain delayed the Programme for Government meeting after the DUP raised objections over the pledge of office and threatened to boycott the occasion.
Mr Kelly told the BBC's Spotlight programme later that evening that discussions were still ongoing.
"This is a very fundamental issue, it always has been for republicans, and we intend to get it right," he added.
Sinn Fein has confirmed that its Ard Chomhairle or party executive will be briefed on the St Andrews Agreement on Thursday in Dublin. The executive is the body which is expected to respond to the government by 10 November.
Last week's St Andrews Agreement stated that before the government legislated on the pledge of office, "it will consider the outcome of further Preparation for Government Committee discussions on policing and the rule of law".
The Northern Ireland parties have been given until 10 November to respond to what the governments are calling the St Andrews Agreement.
It was published after intensive three-day talks between the parties at St Andrews in Scotland.
If all goes to plan, a first and deputy first minister will be nominated on 24 November and the devolved institutions will be up and running by 26 March.