The Ulster Unionist executive has told Mr Jeffrey Donaldson to toe the party line on the Belfast Agreement or face the consequences.
Members of the executive, including Mr David Trimble, voted decisively yesterday by 50 votes to 35 to demand that Mr Donaldson retake the Westminster whip, accept party rules and policies and "desist from any further detrimental activities".
He has until January 9th to respond. Yesterday's move by the executive could be the first step towards censure of the Lagan Valley MP, or even expulsion.
The decision was reached following a four-hour meeting at Ulster Unionist headquarters in east Belfast. The 110-member executive body made its demands on the rebel MP after what was understood to have been a stormy meeting.
However, there was no such demand made of Mr Donaldson's allies on the issue, Mr David Burnside and the Rev Martin Smyth.
Mr Burnside is the South Antrim MP who took the seat from the DUP at the last election and Mr Smyth is the party president and South Belfast MP. Speaking afterwards, Mr Donaldson said the move against him was tantamount to a personal purge and was an attempt to blame him for the party's difficult election showing last month.
He said: "Basically I have been told that I have to accept the pro-agreement policy or I have to leave.
"I believe that this represents not only a clear move against myself but also the anti-agreement members of this party. I believe that this was orchestrated."
Mr Trimble attempted a conciliatory tone. "The party executive very deliberately did not call on Mr Donaldson to resign. It is not our wish that he should do so," he said.
"It is our wish that he should continue to be a member of the party, supporting the policies and the decisions of the party." He added: "We have adjourned the matter to look again in January at these issues."
However, there was no doubt last night that the leadership has moved with resolve and is not going to let the matter rest. One theory is that the leadership is content to keep pressure on Mr Donaldson and his supporters within UUP ranks, but without precipitating a crisis after a difficult election by disciplining him now. Mr Donaldson could push for another meeting of the ruling Ulster Unionist Council and seek wider backing for his stance there. But this could be unlikely given his most recent defeat in a UUC vote on September 6th.
Yesterday's meeting took place amid signs that Mr Donaldson is continuing to step up his opposition to the Trimble leadership.
Mr Donaldson, the Rev Martin Smyth and Mr Burnside resigned the party whip at Westminster last June.
They said they wanted to be free to oppose the Joint Declaration agreed by the British and Irish governments.
The Lagan Valley MP insisted he would consider his position if party policy towards the Joint Declaration did not change.
The Ulster Unionist Council considered the issue on September 6th and backed a leadership motion demanding that all MPs retake the whip and abide by party decisions. However, the three have yet to do so.
In a speech in his native south Down on Thursday night Mr Donaldson dropped further hints that he may consider closer co-operation with the Democratic Unionists.
"I intend to use the mandate I have been given to work with the Democratic Unionist Party, and with other unionists, to achieve the change the unionist people voted for," he told a meeting in Kilkeel Orange Hall.