UUP Assembly member Mr Fred Cobain said that a "disastrous one-way truth commission" would emerge if the North's Police Ombudsman, Ms Nuala O'Loan, was granted extra powers.
Round-table talks involving the two governments and the North's political parties will be held in Belfast today.
The discussions, chaired by the Minister for Foreign Affairs, Mr Cowen, and the Northern Secretary, Mr Paul Murphy, will take place at Stormont. The DUP is boycotting the meeting.
The only anti-agreement unionists present will be the UK Unionist delegation led by Mr Bob McCartney.
Meanwhile, the Ulster Unionists have described the reconvened Forum for Peace and Reconciliation, which met in Dublin yesterday, as a "waste of time". UUP Assembly member Mr Dermot Nesbitt claimed that it was "no more than a publicity stunt".
He said: "The Irish Government, Sinn Féin and the SDLP will likely rake over the same old ground. They will not focus on the central issue that nowhere else in the democratic world, except in Northern Ireland, would anyone be expected to have a government comprising those linked with paramilitarism.
"Republicans are in a deep hole over the recent series of revelations, and what the Irish Government and the SDLP must realise is that, by throwing this forum lifeline to republicans, they are merely aiding and abetting an organisation still wedded to methods and tactics that are far from democratic."
New police reforms tabled by the British government include plans to extend the scope of her investigations. But Mr Cobain warned that tensions could rise if her office was allowed to carry out retrospective probes into alleged wrongdoing by RUC officers over the past 30 years.
"This would be a disaster. People would be calling for inquiries into all security force activity when we need to draw a line under all of this. Look at the problems with Bloody Sunday - it is going to be like that 10 times over. This is going to end up as a one-sided truth commission with unionists being further alienated."