The Ulster Unionist Party's decision to appoint of a joint unionist candidate to fight the Mid-Ulster by-election has resulted in the resignation of former deputy leader John McCallister and outspoken MLA Basil McCrea.
The South Down MLA said last night he strongly opposed the idea of unionist unity and announced he was standing down soon after the Democratic Unionists and UUP revealed that Nigel Lutton was chosen to represent unionism in next month's contest.
Mr McCallister said: "It's nothing personal to Nigel, I know Nigel. It is about principle. I am opposed to unionist unity and I do not want to be part of that."
Mr McCrea announced his intention to resign on BBC Radio Ulster's Nolan show.
He said that the Mid Ulster by-election was "squaring up to be a sectarian battle" and that a joint candidate damaged the "whole of Northern Ireland".
"I don't know what the UUP stands for, what is the difference between them and the DUP?" he said.
Mr McCallister made the decision on the BBC television politics show The View and said he had informed his constituency association.
He was first elected to the UUP in 2007.
He said he had grave concerns about the direction in which the UUP was going.
His departure is another blow for Mike Nesbitt's UUP which has already lost political heavyweights like Ken Maginnis and David McNarry.
Lagan Valley MLA Basil McCrea, who is a close ally of Mr McCallister was given a formal warning from the UUP leadership over comments he made about the handling of the flags crisis.
East Londonderry MLA David McClarty stood as an independent after he was deselected in 2011.
Mr McCallister said he had informed Mike Nesbitt about his resignation earlier.
In a hard-hitting open letter he accused the UUP leader of lacking courage, failing to stand up to the DUP and becoming Peter Robinson's junior partner.
He said: "Your failure to articulate and communicate a distinctive UUP stance - based on Ulster Unionism's core values - on last year's parades controversies, on the Covenant centenary, on the Union flag debate, on the misguided Unionist Forum and on the potential of electoral pacts with the DUP has unfortunately inflicted grave damage on the Party.
"Under your leadership a profound disconnection has occurred between UUP policy and Ulster Unionist values. The policies have increasingly become alienated from the values which should guide and shape an Ulster Unionist.
"Above all, your determination to act in concert with the DUP - over parades, flags and Forum - has significantly contributed to forcing Northern Ireland politics back into the sectarian trenches.
"Rather than building a confident and generous pro-Union centre ground, you have opted instead to become Peter Robinson's junior partner."
Mr McCallister, who is married with a young family, said he had first voted for the UUP in 1992 but was now unhappy with the direction it was going.
He said he took no pleasure from making the criticisms.
But, he added: "Unionist unity is an exercise in the politics of tribalism, declaring to voters that our society is forever divided between orange and green blocs.It states that all other political issues and debates can be sidelined, because tribal identities must determine our politics.
"I find it a matter of great personal sadness that the party which I have supported since 1992 is now acting in a manner which deepens and intensifies the divisions in our society."
Mr McAllister said he intended to sit as an independent MLA in the Assembly.