A supporter of Ulster Unionist leader Mr David Trimble claimed today the independent body for the Belfast Agreement was "fatally flawed".
In a blow for the UUP leader ahead of a crucial party meeting in Belfast tomorrow, former Mid Ulster Assembly member Mr Billy Armstrong said proposals for the ceasefire monitoring commission offered "no genuine means" of excluding from government those parties with paramilitary links.
His warning came as three rebel MPs sent a letter to 900 members of the Ulster Unionist Council warning of the potential of an "irreversible" split in the party.
"I had hoped that the ceasefire monitoring body could have provided an effective means of holding Sinn Féain/IRA to account, providing sanctions against their continued terrorist activity," Mr Armstrong said.
"The draft agreement between our government and the Republic's government is fatally flawed, offering no genuine means of excluding parties who have continuing links to terror groups.
"The governments have failed - yet again - to address the cancer at the very heart of the political process. There can be no accommodation between democracy and terror. Republicans must democratise or be excluded from government," he said.
Mr Armstrong has supported his leader in several battles with the Donaldson wing of the party since 1998. UUP members will be watching closely to see if it marks the start of a stampede of other members of the Assembly group, once a loyal powerbase for Mr Trimble, against him.
Three rebel Ulster Unionist MPs today warned party leader Mr David Trimble he is risking a "disastrous and irreversible split" in the party by backing disciplinary action against them.
In a letter to 900 members of the Ulster Unionist Council, Mr David Burnside, Mr Jeffrey Donaldson and the Rev Martin Smyth warned there could be a confrontation between their three constituency associations and the party if the leadership continued to try to punish them for resigning the whip.
Supporters of the three MPs have forced a meeting of the council in Belfast tomorrow.
The MPs said: "If this disciplinary process continues, it holds the prospect of a disastrous and irreversible split in Ulster Unionism which will leave the party as a minority rump at Westminster and no longer the major party in any future Assembly or local administration in Northern Ireland.
"Let us state unequivocally that this is not an outcome that we wish to see happening to our party."
The MPs angered the party hierarchy in June by resigning the whip because of the Ulster Unionist Council's failure to completely reject proposals from the British and Irish governments on the future of the peace process.