The three MPs who resigned the Ulster Unionist whip at Westminster were challenging the "character, objectives and leadership" of the party, and should quit, the party leader said yesterday.
In a statement, Mr David Trimble accused Mr Jeffrey Donaldson, Mr David Burnside and the Rev Martin Smyth of siding with "other political elements" which have "amply proved their inability to offer constructive leadership to the unionist electorate and have as their primary objective the destruction of the UUP".
Accusing the three of a direct repudiation of the Ulster Unionist Council decision taken on June 16th, he asserted that the positions of the president and the vice-president of the UUC - Mr Smyth and Mr Donaldson - were now untenable.
He said resignation was the only honourable path open to them, but said they were leaving it up to the rest of the party to "tidy up the situation".
He confirmed he had called on the UUP chairman to convene a meeting of the party officers to deal with the status of the three rebels.
Borrowing one of Mr Donaldson's favoured phrases, Mr Trimble said the UUP and unionism as a whole were facing "a defining moment".
"At stake is all the progress that has been made in recent years. We are working to deliver, within the Union, a peaceful democratic Northern Ireland, at ease with itself and with its neighbours."
Mr Trimble said he hoped he had witnessed "the last effort to turn the clock back", and looked to the UUP being free to complete its "historic endeavour".
Speaking later, Mr Trimble went further in his criticisms of the three.
"They have shown they have no capacity for constructive leadership, and seeking alliances with people who have failed to deliver things is completely the wrong way to go."
He said that view would be taken by the party's executive just as it had been adopted by the UUC last week.
He denied his leadership was fatally damaged, and shrugged off suggestions that, having lost half his parliamentary party, he should be the one to quit.
"Over the course of the last number of years I have done my best to hold the party together, to keep people on a reasonably even keel and to work through the party processes. Some people have decided they cannot do that any longer; the responsibility for their actions lies unambiguously with themselves," he told the BBC.
He added he had given his critics every chance, but the opportunities offered had been spurned. "I don't think the party at large will have any difficulty in identifying the people who have behaved unreasonably."