The SDLP's Brexit spokesperson has resigned from her position and has also stood down as a member of the SDLP's Assembly group.
Clare Hanna was opposed to the party's decision to develop a closer relationship with Fianna Fáil.
Ms Hanna argued against the move.
She is to remain an SDLP member and will continue in her role as a South Belfast representative of the currently closed Northern Ireland Assembly.
It is the latest in a series of resignations from the party since the decision was made.
The chairpersons of the party's youth, women and LGBT+ wings said they were stepping down in an open letter that was sharply critical of how the decision was reached.
On Saturday, leader Colum Eastwood hailed the birth of a policy partnership he said would build for a better future in the years ahead.
Matthew Corr (youth), Caoimhe McNeill (women) and Matthew Carson (LGBT+) wrote: "Calls for respect and party unity ring hollow when a coach and horses has been driven through the proper procedure which exists to facilitate a debate, on equal terms, and allow the party to come together after."
They added Saturday's conference was called without proper notice being given.
"Voting opened while the debate was ongoing, allowing people to vote before they had heard all points of view," the letter said.
"This was extremely discourteous to speakers of every side.
"There can be no reasonable debate when the terms of the debate are arbitrary and extra-constitutional."
The party members said they would campaign for SDLP candidates in May's local government elections.
"Our commitment to electing progressives who can make a difference to the lives of our people remains unwavering," the said.
"But a line must be drawn - party members will not be treated like this without consequence."