The decision by the three Ulster Unionist MPs to quit the party's whip at Westminster is welcome but does not go far enough, the DUP has said.
In a statement, the party leader the Rev Ian Paisley called on Mr Donaldson, Mr Smyth and Mr Burnside to face up to what he said was an "incompatibility" of the three anti-agreement MPs remaining inside a pro-agreement party.
In a clear call for them to go further, Dr Paisley added: "Those who have fought to change the direction of the Ulster Unionist Party must realise that by remaining members of that party they are in actual fact helping to sustain David Trimble and the agenda he is pursuing. Anti-agreement Ulster Unionists will want to address this issue."
In a wider call for UUP defections, Dr Paisley said: "There has been a battle within Ulster Unionism over recent years, boldly fought by those who oppose David Trimble and who still adhere to unionist principles. Yet it has ended in repeated defeat and there is no realistic prospect of turning that around.
"The Ulster Unionist Party has effectively endorsed the Joint Declaration. It is more pro-agreement today than it has ever been." He claimed that Mr Trimble and the remaining two UUP MPs who support him at Westminster were now outnumbered by nine unionist MPs who "adopt a traditional unionist stance".
He continued: "The momentum and direction of mainstream unionist opinion is clear. A very clear majority of unionists now share the common view that the Belfast Agreement and its offspring, the Joint Declaration do not provide the basis for political stability in Northern Ireland."
The Assembly party which represents UUP members of the suspended Stormont institution said it regretted yesterday's developments.
"It is for us a matter of deep regret that three of our Westminster colleagues find themselves unable to accept party policy as agreed at the UUC meeting less than a week ago by a clear majority. We were told that last Monday's meeting was a defining moment. Instead, by their actions, our colleagues have shown only contempt for UUC delegates."
In a statement, they added: "The agreed aim of the UUP is to bring about a resolution of our historic conflict on a democratic basis. That task is made all the harder when fellow Ulster Unionists fail to uphold democratic decisions." They denied they endorsed uncritically the two governments' Joint Declaration. "Final decisions on that document must await acts of completion from the republican movement".
Sinn Féin yesterday accused Mr Trimble of allowing the No wing of Ulster Unionism to set the agenda.
Denying the issues were about competing personalities, Mr Mitchel McLaughlin, the Sinn Féin chairman, said the hiatus within the UUP had more to do with "fundamental issues facing unionism at this time".
He added: "If unionism proves incapable of managing the process of change then the two governments, and the British government in particular, must act to implement the Good Friday agreement and hold elections to the Assembly."