The chairman of the SDLP in east Derry said yesterday his branch members felt betrayed and baffled by the decision of an SDLP councillor in Coleraine to resign from the party and join Sinn Féin.
Mr Billy Leonard, a member of Coleraine Council, is a Protestant and a former RUC reservist.
He resigned from Coleraine's District Policing Partnership group before announcing he had switched his political allegiance to Sinn Féin.
SDLP branch chairman Mr John McCauley accused Mr Leonard of putting his personal political ambitions before his principles, and called on him to resign his council seat.
Mr McCauley said: "I was baffled by Billy's decision and disappointed. I spoke with him last Thursday about party business and about increasing the popularity of the SDLP in Coleraine and he made no mention of his impending decision to join Sinn Féin. Either he made his decision overnight or it was something he had been thinking about."
Mr Leonard said he had thought long and hard before deciding to join Sinn Féin and becoming the party's first Coleraine councillor.
"I would say in terms of the political position, I feel happier in Sinn Féin, but I am not passing any personal comment on the leader of the SDLP, Mark Durkan. I am at the position now where I am a member of a party which I believe best represents the changing political tide within nationalism."
Mr Durkan commented: "In my recent conversations with Billy Leonard, he did not raise any concerns about the leadership and direction of the party. In fact, in my last conversation with him before his defection, what Mr Leonard raised was his chances of being appointed a special adviser by an SDLP minister when the institutions were restored."
The chairman of Sinn Féin, Mr Mitchel McLaughlin, said his party had no problem in accepting as a member a former member of the RUC. "I met Billy Leonard to discuss this move. I received an extensive briefing from him and from his friends on his personal and political background. He has dealt with us in a very open and frank manner and our party has dealt with him in a similar vein."