Senior Ulster Unionists have decided to refer three rebel MPs to a disciplinary committee.
Three Ulster Unionist MPs quit the party whip at Westminster
The emergency meeting on Thursday followed the decision by Jeffrey Donaldson, David Burnside and the Reverend Martin Smyth to resign the party whip at Westminster.
The three MPs had also called for party leader David Trimble to change party policy or step down from the post.
In response, Mr Trimble said the so-called "gang of three" should quit the party.
The motion was passed by five votes in favour of the disciplinary committee with two voting against.
Speaking after the meeting, Mr Trimble said it had not been a good week for unionism.
"I think the actions taken by the three gentlemen on Monday have triggered a crisis within the party and have left the party with no alternative but to respond in the way that it has," he said.
He added: "We can't be expected to indefinitely put up with the situation where there is a
party within a party."
Mr Donaldson said he regretted the decision to refer them to a disciplinary committee.
He said the move against the three MPs was in effect a move against almost half the Ulster Unionist Party.
"The party is no longer a broad church," he said.
"The decision will do nothing to address the problems within unionism."
Later on Thursday, Ulster Unionist Arlene Foster resigned from her post as party officer.
Ms Foster, who is one of the party's honorary secretaries, said she could not be complicit in any action taken against three men who "have shown exceptional dedication to the Unionist party and which are the actions of a vindictive leader who has become more detached from the people who elected him".
Mr Smyth and Mr Donaldson are facing a charge of failing to implement decisions of the Ulster Unionist Council, in their capacity as party officers.
All three MPs have been charged with breaking an undertaking which they signed when they stood for election to take the party whip, and with bringing the party into disrepute.
The disciplinary committee will be chaired by solicitor and party member Raymond Ferguson.
Any decision by the disciplinary committee to expel the three MPs would have to be ratified by the 110-strong Ulster Unionist executive.
Trimble supporters are in the majority amongst the 14 party officers.
Last week, Mr Trimble narrowly fended off a challenge from party rebels, led by Mr Donaldson, who wanted the party to reject the recent British and Irish joint declaration.
Mr Trimble, who criticised some elements of the declaration, whilst noting that the party had not accepted the document as a whole, secured 54% of the vote at a meeting of the party's ruling council. Mr Donaldson received 46%.
Sceptics within the party claim the rebels represent the true spirit of unionism and have vowed to fight any move to expel or suspend them.
The three MPs are to begin talks with the DUP and UK Unionist Party to adopt a united front against the joint declaration and press for fresh negotiations on the way forward.
Mr Donaldson said he had considered leaving the party but believed taking action at Westminster was in the wider interests of unionism.