A former member of the DUP Executive has defected to the Ulster Unionist Party.
Andrew Muir is unhappy about a possible SDLP merger
Edward Hanna accused his former party of "misleading the electorate and of being arrogant".
Meanwhile, it is understood that the chair of the SDLP Bangor branch in north Down, Andrew Muir, is about to defect to the Alliance Party.
It is thought Mr Muir is concerned the party may merge with Fianna Fail or Labour in future.
In a statement the DUP said Mr Hanna had been "demonstrably in agreement with" party policy.
"The DUP is a strong party, with a growing membership, unlike the sinking ship that Mr Hanna has decided to jump on," he said.