The Ulster Unionist security spokesman, Mr Ken Maginnis, has said the British government could set Northern Ireland policing back 30 years if it implements the Patten report and drops the name of the RUC.
The Northern Secretary, Mr Peter Mandelson, is expected soon - possibly this week - to announce his backing for almost all Mr Chris Patten's proposals in his review of policing.
Mr Maginnis said: "We have cautioned the Prime Minister that implementation of Patten will provide the basis for the sort of dissension that existed 30 years ago."
It is understood that at a recent meeting Mr Blair told Mr Trimble, the First Minister, that he intended putting most of the report into operation soon. Unionists criticised the report when it was published last September.
The report aims to encourage more Catholics to join the police force, which is 92 per cent Protestant, and to make it more acceptable to both communities. The proposals to change the RUC's name to the Northern Ireland Police Service is bitterly contested. The Police Federation has handed in a 400,000-signature petition to Downing Street, supported by the families of officers killed by republicans.
Mr Maginnis said: "There is absolutely nothing to suggest that a name change will bring any benefit to any section of society in Northern Ireland in terms of either quality of policing or recruitment across the two traditions.
"The government now appears to be capitulating to Irish-American pressure and ignoring the opportunity for agreed progress within Northern Ireland."
Moving ahead on the Patten report could increase Mr Trimble's difficulties in trying to keep most of his party behind him when he faces a crucial Ulster Unionist Council meeting next month.
While he won 58 per cent backing from the UUC in November for his proposal to form a power-sharing government with Sinn Fein without prior Provisional IRA decommissioning, he has pledged to resign if decommissioning has not started by February.
The Patten report also proposed cutting the RUC's numbers from about 13,000 to 7,500 and disbanding the 3,000-member reserve. The RUC Chief Constable, Sir Ronnie Flanagan, has said he will resign if the force's capacity to defend the public is curtailed before the security situation allows.