South Belfast MLA Claire Hanna has resigned from the SDLP's assembly group following the party's partnership with Fiann Fáil.
Ms Hanna said she remained "unconvinced" that the partnership "is the right vehicle with which to deliver the non-sectarian, transparent and social democratic new Ireland I believe in".
She added that it had not been an easy decision to make and she had "thought long and hard about the outcome and implications".
Ms Hanna will continue to represent South Belfast as an SDLP assembly member.
SDLP Leader Colum Eastwood said it was "with regret" that the party accepted Ms Hanna's resignation.
"At the SDLP Special Conference all members were afforded the opportunity to speak and delegates to vote on two motions. The SDLP membership overwhelmingly voted for the party to embark on a partnership with Fianna Fáil," he said.
"It is with regret that we accepts Claire’s resignation. The party membership has democratically spoken and difficult though that decision may be for some, their wishes must be respected and enacted."
Ms Hanna has been critical of the policy partnership which was endorsed by SDLP members at a special meeting on Saturday.
She was the only member of the SDLP's assembly team who did not sign a letter urging the party to back the proposed partnership with Fianna Fáil.
She previously told The Irish News she regards the new north-south relationship as a "merger" that will "narrow" her party's appeal at the polls.
I’ve resigned as SDLP Brexit Spokesperson , remaining a party member. pic.twitter.com/TFk2CUUlcx
— Claire Hanna (@ClaireHanna) February 11, 2019
