One of the SDLP's most high-profile MLAs has resigned as the party's Brexit spokesperson in protest at its new partnership with Fianna Fail.
Claire Hanna also said she will no longer go to SDLP group MLA meetings. Contrary to some media reports, she has not resigned the party whip.
However, the SDLP leadership will now have to decide whether it removes the whip from her, given her cited refusal to attend Assembly party meetings. It is understood this is a "grey area" in the party's constitution.
The South Belfast MLA will remain a party member. Her husband, Donal Lyons, is a sitting SDLP councillor and is standing in May's local government election in the Balmoral ward.
Ms Hanna was a vocal critic of the link-up with Micheal Martin's party. Before the vote at last weekend's special SDLP conference in Newry, she said she would never be a Fianna Fail MLA.
Announcing her decision on Twitter yesterday, she said: "Following the conference on Saturday, I have thought long and hard about the outcome and implications.
"I remain unconvinced that an exclusive partnership with Fianna Fail is the right vehicle with which to deliver the non-sectarian, transparent and social democratic new Ireland I believe in.
"As such, after a discussion with Colum Eastwood, I have resigned from my position as SDLP Brexit spokesperson. This hasn't been an easy decision."
She added: "I remain an SDLP member and MLA, and will be continuing to work hard for the people of South Belfast and alongside SDLP members and activists, with whom I still share political values and aspirations."
Ms Hanna was seen by some as a future SDLP leader.
The Fianna Fail link-up was endorsed by 70% of delegates at the special party conference.
The South Belfast MLA supported a defeated motion that opposed the exclusive relationship with Fianna Fail and instead advocated a broader approach to political collaboration with parties in the Republic including Fine Gael and Labour.
Reacting to Ms Hanna's decision, Colum Eastwood said: "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 Fail. It is with regret that we accept 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."
On Saturday, Mr Eastwood hailed the birth of a policy partnership he said would build for a better future in the years ahead.
But within 24 hours, the chairpersons of the SDLP's youth, women and LGBT wings said they were stepping down in an open letter that was critical of how the decision was reached.
In a statement, Matthew Corr (youth), Caoimhe McNeill (women) and Matthew Carson (LGBT) claimed they took the "only course of action" available to them after "procedural abuses" at the special conference.
In a letter to party chairman Colin McGrath, they expressed concern at the way the meeting was convened, the process to submit motions or amendments, and the lack of notice given to members.
Despite the scathing attack, Mr Corr said all three members would continue to "campaign hard" for party candidates during the council elections in May.
The party rejected their claims that proper procedures were not followed at the special conference.