Three rebel Ulster Unionist MPs were tonight told to toe the party line after they rejected an attempt to get them to resume the whip at Westminster.
Ulster Unionist chairman
Earlier today, Mr Donaldson insisted UUP Westminster chief whip Mr Roy Beggs was trying to impose an "unrealistic demand" on the trio, who resigned the whip in June in a row over party policy.
Expressing his sorrow at Mr Donaldson's remarks, the UUP chairman insisted every effort was being made to accommodate the three MPs.
"We believed that we had put the whole episode behind us," he said.
"It is sad that some within the party still stand defiant of majority party wishes."
Ulster Unionists had hoped that when the party's 110-member executive recently rejected most of the Irish and British Governments' plans for the peace process that divisions would have healed.
But as negotiations between Sinn Féin president Mr Gerry Adams and Ulster Unionist leader Mr David Trimble reached a critical stage, the three MPs mad e their presence felt by insisting the IRA must end and all paramilitary activity before they would endorse power sharing with Sinn Féin.
Mr Cooper said tonight it was "wrong to suggest that the three MPs are being asked to do too much in order to retake the whip.
"Rather, the chief whip Roy Beggs has simply asked them to abide by future whipping decisions consistent with relevant policy decisions of the Ulster Unionist Council and its executive committee.
"For any political party this is a normal undertaking," he said.
Mr Donaldson was defiant, claiming some UUP members were concluding the leadership was "pursuing a vendetta" against him and his colleagues.
He said: "Our concern is that we are being asked to agree future policies which we have not seen.
"I have to say we will not take lectures from Mr Cooper about obeying party rules when he and other officers were told by a High Court judge that they did not abide by party rules in trying to suspend us.
PA