Anti-Agreement Ulster Unionist MP Jeffrey Donaldson has been ordered to support the leadership of the Ulster Unionist Party or face expulsion.
A motion was passed at the party's 108- strong executive telling Mr Donaldson to back party leadership David Trimble or resign from the party.
An angry Mr Donaldson tonight described it as a purge on the anti-Agreement wing of the party and blamed an orchestrated campaign of letter writing aimed at forcing him out.
The resolution agreed tonight stated that Mr Donaldson's conduct during the Assembly election campaign had been detrimental to the interests of Ulster Unionism and to the UUP.
It was passed by 55 votes to 33 with four abstentions after a stormy meeting which lasted more than three hours.
Mr Donaldson said: ``I believe that this represents a clear move against not only myself but against the anti-agreement members of this party.
``I believe this was orchestrated today, there were letters that came in which were repetitious and clearly that had been part of a campaign. This was quite deliberate. It was designed to target me specifically, to blame me for the poor election result of the Ulster Unionist Party.''
Tonight's move follows Mr Donaldson's calls on David Trimble to resign after the party was overtaken by Ian Paisley's hardline DUP in last month's Assembly elections.
The Ulster Unionist MP now has a period of three weeks to support the party leadership or face disciplinary action.
Observers believe it could now be all over for Donaldson and his allies in the UUP, and that he may carry out his recent threat to join the DUP.
Earlier, Donaldson claimed Ulster Unionist leader David Trimble was pushing his party into ``the second division'' of northern politics.
He told Mourne Unionist Association's annual meeting: ``It is clear that the leadership has little to show for the gambles they have taken with the integrity of our party.
``Now we have been overtaken by Sinn Fein/IRA for the first time in our history in terms of first preference votes cast at a major election. This represents a humiliation for Ulster Unionism.''
``Yet, in spite of all this, there is no indication that the leadership will change the policy of the party to reflect the new political dispensation in unionism. Thus we find ourselves in the same position as the Conservative Party in 1997 - no change, no chance!''
The DUP secured 30 seats in the Belfast Assembly, with the Ulster Unionists taking 27. The Ulster Unionists finished behind the DUP and Sinn Fein in the popular vote.
Mr Trimble denied that there was a conspiracy to purge Mr Donaldson from the party.
He said: ``The Executive very deliberately didn't call on Mr Donaldson to resign. It isn't our wish that he should do so. It is instead our wish that he should continue to be a member of the party supporting the policies and the decisions of the party.''
Sinn Féin Assembly member Alex Maskey has said that two weeks on from the election `neither Jeffery Donaldson or the DUP have yet produced a credible plan for the future'.
``The two governments now have a crucial role to play in all of this,'' he said. ``They must move ahead with a pro-Agreement agenda and make it clear to the rejectionists that they will not be allowed to block forward movement.
``Donaldson and the DUP need to recognise that the vast majority of people who voted in the election voted to support pro-Agreement parties and they returned a majority of pro-Agreement MLAs to the Assembly.''