SDLP assembly member Annie Courtney has said she is quitting the party and will sit as an independent on Derry City Council.
Mark Durkan said he regretted Mrs Courtney's decision
It follows her deselection as an assembly candidate at a party meeting in February.
She said she had not decided if she would contest her assembly seat in the forthcoming elections at the end of May.
SDLP leader Mark Durkan said on Tuesday that he regretted Mrs Courtney's decision.
He said the SDLP had difficult choices to make, but that he wished her well for the future.
The Foyle assembly member said she had considered her decision deeply.
"I suppose it's a decision I wouldn't be taking lightly.
"The amount of stress and pressure that I have personally been under is unbelievable," said Mrs Courtney.
"This has also affected my family - not only my immediate family but the wider circle and they have probably got to the stage where they didn't want to see me under any more stress or hurting more.
"So, for that reason I am stepping aside," she said.
Mrs Courtney was angry at being deselected to stand for the assembly and described the February meeting as a "sham and a farce" because of the method the party used to choose its candidates.
SDLP councillor Gerard Diver, from the Waterside area of Londonderry, replaced Mrs Courtney on the party's election team.
The other candidates are Mr Durkan, Derry Mayor Pat Ramsey and councillor Mary Bradley.