More than 30 members of Mr David Trimble's constituency association walked out in a mass resignation at the weekend.
They resigned at the annual meeting of the Ulster Unionist Upper Bann Association, the scene of a no-confidence vote against the party leader last July.
The resignations show a party still in turmoil and follow a large-scale defection of Ulster Unionists in neighbouring Lagan Valley, where the sitting MP, Mr Jeffrey Donaldson, switched to the DUP earlier this month.
Mr Garfield Gilpin, a former vice-chairman of the Upper Bann association, was among the delegates who quit on Saturday night.
He warned: "A large section of the party is removing itself from the party,and if enough leave, the party will collapse, because economically it will not be viable either in votes or in financial terms," he said.
"[Mr Trimble] has got away from basic Ulster Unionist principles and no longer represents the majority of grassroots traditional unionism. The wheels are falling off the machine. We are probably looking at several branches facing closure."
The resignations have taken place against a background of continued murmurings within the party about Mr Trimble's leadership. The Ulster Unionist leader must put himself for re-election at each agm of the ruling Ulster Unionist Council.
The next such meeting is in March, and speculation persists that there could be a move among the party's top-brass to ask Mr Trimble to stand down.
Three Assembly members, Mr Donaldson, Ms Arlene Foster and Ms Norah Beare, have already left the party for the DUP. Mr Ivan Davis, a Trimble loyalist and a former party chief whip also stood as an independent in the November election in response to his de-selection as a candidate in Lagan Valley.
Supporters of Mr Trimble and Mr Davis in Lagan Valley faced disciplinary action by elements within the UUP who are hostile to Mr Trimble's "failed" policy on the Belfast Agreement and the Joint Declaration. Mr Davis and three councillors who supported him, Mr Jim Dillon, Mr David Archer senior and Mr Sam Johnston, were accused of acting in a manner "disadvantageous to the best interests" of Ulster Unionism.
The Ulster Unionists have lost their position as the largest and most influential unionist group. They now have 24 seats in the suspended Assembly and five seats at Westminster. The DUP commands the unionist benches in Stormont with 33 members and now has six MPs.