South Down MLA John McCallister has announced that he has resigned from the Ulster Unionist Party.
He said he informed Ulster Unionist leader Mike Nesbitt of his decision on Thursday, and has also told his constituency association.
The move came just after the UUP and DUP endorsed victims campaigner Nigel Lutton as their joint Unionist candidate to run in the Mid Ulster by-election next month.
The TUV and Willie Frazer also lent their support to Mr Lutton.
However Mr McCallister made his announcement on BBC Northern Ireland’s politics programme The View, following the official announcement of the joint Unionist Mid Ulster candidate.
He told the BBC programme: “It is therefore with immediate effect and deep regret that I resign the whip of the UUP and my Party membership”.
Mr McCallister added that he plans to sit as an independent unionist in the Assembly.
In a letter to Mr Nesbitt, Mr McCallister said he had first voted for the UUP in 1992, because of the party’s values and had “no interest in supporting other narrow expressions of unionism, committed as they were to confrontation and triumphalism”.
“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,” he wrote.
“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 leader.
“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.
“At a time of division and uncertainty in our society, Northern Ireland needed the UUP to set out with courage a pro-Union alternative to the politics of sectarian headcounts.
“It gives me no pleasure to say that, under your leadership, the UUP has utterly failed to do so. Rather than building a confident and generous pro-Union centre ground, you have opted instead to become Peter Robinson’s junior partner.”
Mr McCallister said that Mr Nesbitt’s “unwillingness” to advance the cause of an opposition in the Assembly had similarly damaged both the UUP and Northern Ireland politics.
“You have taken no action or measures to move towards the creation of an Opposition, offering instead merely half-hearted words,” he said.
“The creation of an Opposition would not only offer genuine accountability in the Assembly. It would also offer an alternative to the politics of the sectarian headcount propagated by the DUP and Sinn Fein.
“Your failure to act on this issue has resulted in a UUP incapable of meaningfully challenging the DUP’s current electoral position. “
Mr McCallister also accused Mr Nesbitt of having “abandoned pluralist and progressive pro-Union politics for a backward-looking, insular politics that is in the interests of neither Northern Ireland nor the Union”.
“The decision to repeat a failed electoral strategy and run a ‘unionist unity’ candidate in Mid-Ulster demonstrates the extent to which you have decided to abandon any pretence that your leadership can make the UUP a home for pluralist and progressive pro-Union politics,” he said.
“’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.”
An Ulster Unionist press officer told the News Letter on Thursday night that the party would not yet be commenting on the resignation.
Mr McCallister was first elected to South Down in 2007, following the retirement of former Ulster Unionist MLA Dermot Nesbitt.