Ulster Unionist Party leader Mr David Trimble launched moves tonight to have his party president, the Rev Martin Smyth, kicked out of the post after he joined two other hardline MPs in resigning the party whip in Westminster.
He is also seeking to have Mr Jeffrey Donaldson removed from his position as one of the party vice presidents.
Mr Trimble's announcement came after consultation with lawyers following the resignation of the whip by Mr Smyth, Mr Donaldson and Mr David Burnside in opposition to his peace process policy and particularly the two governments' Joint Declaration on the process.
They made their move earlier today after their failure a week ago to get the ruling Ulster Unionist Council (UUC) to back their demand for a total rejection of the Declaration on the way forward for restoring devolution at Stormont.
Mr Trimble tonight accused the three renegades of making it clear "they will not accept party democracy".
He said the reasons they gave for their action were "disingenuous in the extreme".
He added: "After such a direct repudiation of the Ulster Unionist Council's decision, the positions of its president and one of its vice presidents are wholly untenable."
Resignation would clearly be the "principled course of action" he said, adding that in any party elsewhere in the UK their actions would be regarded as a resignation from the party.
Mr Trimble said he could only assume they "recognise and intend this outcome, but merely wish to place on the rest of the party the task of tidying up the situation.
"So be it. I have therefore asked the party chairman to convene a special meeting of party officers later this week."
Mr Trimble said it was "a defining moment not just for the Ulster Unionist Party but for unionism as a whole."
Making clear he was not going to lie down in face of the hardline anti-Agreement opposition from within his ranks he said it was also a key moment for society in Northern Ireland.
"At stake is all the progress that has been made in recent years."
The party was working to deliver a peaceful, democratic Northern Ireland at ease with itself and its neighbours, said Mr Trimble.
The UUC challenge by the hardliners was the latest in a series of failed attempts to change Mr Trimble's stance. Of the latest move he commented: "This is the latest, and I hope the last, effort to turn the clock back.
"It may also free the party to enable it to complete successfully this historic endeavour".
PA