SDLP leader Colum Eastwood last night voiced regret at Claire Hanna's decision to resign from her role as the party's Brexit spokeswoman.
The South Belfast MLA also left the party's assembly group yesterday after a majority of members had endorsed a formal partnership with Fianna Fáil.
Delegates voted by 121 votes to 53 to support Mr Eastwood's proposal for greater links with the Republic's main opposition party.
Ms Hanna's unilateral withdrawal of the whip was not entirely unexpected given her vocal opposition to cementing formal ties with Fianna Fáil.
At Saturday's meeting in Newry she supported a defeated motion that had opposed the exclusive relationship with Micheál Martin's party, advocating instead a broader approach to political collaboration south of the border, incorporating engagement with Fine Gael and Labour.
However, she insists that she will remain an SDLP member and MLA.
In a statement issued yesterday afternoon, she said she had thought "long and hard" about the implications of Saturday's vote.
I’ve resigned as SDLP Brexit Spokesperson , remaining a party member. pic.twitter.com/TFk2CUUlcx
— Claire Hanna (@ClaireHanna) February 11, 2019
"I remain unconvinced that an exclusive partnership with Fianna Fáil is the right vehicle with which to deliver the non-sectarian, transparent and social democratic new Ireland I believe in," she said.
"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."
Ms Hanna, whose husband Donal Lyons is also a Belfast SDLP councillor, said she would continue to "work hard" for her constituents and alongside party members and activists "with whom I still share political values and aspirations".
Her resignation comes after the chairs of the party's youth, women and LGBT+ wings said they were stepping down amid criticism of how the weekend's special meeting was conducted.
Mr Eastwood said Saturday's conference gave all members the opportunity to speak, and for 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."
Former SDLP agriculture minister Brid Rodgers said she was disappointed Ms Hanna had resigned as the party's Brexit spokeswoman.
"It's very sad to see us lose one of the most articulate and well-informed voices on the Remain side – Claire has been outstanding in that role," she said.
The former Upper Bann MLA, who was among six former party chairs who proposed a motion at the end of the meeting to have the leadership's proposal "referred back" – or rewritten – declined to comment on her colleague's motivations for resigning.
Meanwhile, the Fianna Fáil leader has welcomed the SDLP's endorsement of a partnership.
"I was obviously pleased by the emphatic support within the membership of the SDLP for the partnership between our two organisations," Mr Martin said.
"I look forward to working with Colum and Nichola (Mallon) and the team in the years ahead to develop joint policies that begin to tackle the real issues that are impacting on people’s lives but are being completely ignored in the current dysfunctional political environment."
