David McClarty has rejected an offer to rejoin the Ulster Unionists - a move that deprives the party of a second Stormont ministry and heightens the pressure on Tom Elliott's leadership.
A special meeting of Mr Elliott's party executive has been arranged for Saturday to discuss the deepening crisis and Mr McClarty's decision to remain an independent is set to strengthen his critics' case for a change at the top.
Mr McClarty, who left the UUP after failing to be selected as an Assembly candidate, said he would have lost respect if he had made a U-turn days following his electoral triumph in East Londonderry.
"I sought election as an independent, all my literature was as an independent and people voted for me as an independent," he said. "I think I would lose all credibility if I snubbed my nose at those people and not remain as an independent as they elected."
His much-anticipated decision will see the Alliance Party take the final seat round the executive table.
The Ulster Unionists suffered losses in both the Assembly and local government elections, prompting questions about Mr Elliott's future.
And the damage at the polls was arguably intensified by controversial comments made by him in the wake of his election in Fermanagh and South Tyrone when he branded Sinn Fein as "scum" during his acceptance speech at Omagh Leisure Centre.
A direct challenge to his leadership is unlikely to materialise at Saturday morning's executive meeting in Dungannon, in the heart of his own constituency, but one could potentially emerge before then.
But Mr Elliott, who insisted he had always planned to hold a party executive meeting after the election and it was not "out of the ordinary", said he did not envisage a challenge.
"We had a meeting of the Assembly group yesterday and there was no hint of anything like that," he said. "And I don't see anything like that happening."