The British government was warned tonight it will have to amend its police reforms for Northern Ireland if it is to persuade unionists to join the board which will handle them.
Leading anti-Belfast Agreement Ulster Unionists Mr Jeffrey Donaldson and Ms Arlene Foster told Northern Ireland Secretary Dr John Reid he would have to satisfy their demands if he is to secure the party's involvement on the 19-member Policing Board.
"The ball is in his court," Mr Donaldson warned after the UUP executive deferred a decision on whether to join the board.
"He is the person who can deliver the assurances we need, the changes that we need."
The Ulster Unionist Executive unanimously backed a motion reaffirming its opposition to the Patten Report, which laid the foundations for the reforms.
The motion also called on the British government to address a number of concerns.
Sources at today's meeting said pro and anti-Agreement unionists agreed to set aside their differences on whether to join or boycott the board until they saw how the British government would address their concerns.
Afterwards, UUP leader Mr Trimble said several members were alarmed at the downsizing of the police, citing the summer street disturbances in North Belfast between loyalists and republicans.
The Upper Bann MP said there was also concern about Her Majesty's Inspectorate of Constabulary's report which warned there was "serious danger" that the new arrangements would not deliver effective policing.
"If with regard to any other constabulary in the United Kingdom, HMIC had given a report saying that there is a serious danger that within the course of the next few months we will not have an effective police force, then that would be a matter which would require urgent attention and measures would be proposed and adopted.
"I want to know what the (British) Government, the Northern Ireland Office, is prepared to do to assure the public in Northern Ireland that there is going to be an effective police service which we want to see," he said.
The outcome of today's meeting means only the nationalist SDLP has indicated so far it will participate in the new policing structures.
The rival Democratic Unionist Party and anti-Belfast Agreement UUP members want Mr Trimble to withhold nominations to the board until the British government amends the reforms to their community's satisfaction.
Sinn Féin has indicated it is not going to take its seats on the board.
Mr Donaldson said his party was determined "at every level" to ensure the reforms were amended.
"We are not going to let the people of Northern Ireland down," the Lagan Valley MP told BBC Radio Ulster's Inside Politics programme.
"We are not going to sell them short on policing."
Ms Foster also rejected British government claims that the police reform package was "nonnegotiable".
"The SDLP were able to secure changes they needed from the original implementation plan before they announced they would join the board," she told reporters.
"Is the (British) government seriously telling us that the Ulster Unionist Party, the largest party in Northern Ireland, cannot have its concerns addressed in the same way?"
PA