The SDLP has said the party whip has been removed from the North Antrim MLA Declan O'Loan following his call for a single nationalist party.
Mr O'Loan will now not be able to meetings of the party's assembly group.
It said party leader Margaret Ritchie took action following a meeting with Mr O'Loan on Tuesday.
The party said the whip has been removed for an indefinite period which the leader will decide.
The move means he cannot vote as an SDLP member in the assembly.
On Monday, Mr O'Loan withdrew a call for the formation of a single nationalist party after there was a furious reaction from colleagues.
Earlier on Monday, in a statement not issued through the party press office, he said he had discussed the idea with the grassroots members in north Antrim.
Mr O'Loan said that it was very strongly supported.
But he later withdrew the original statement saying it "does not represent established party policy".
Earlier the North Antrim assembly member said: "I believe that a major realignment of northern nationalism is now called for and I think that this means the formation of a new single nationalist party.
"The values of the SDLP and its ability to command respect and at least second preference votes across the whole community are not something that should be lost.
"In the interests of achieving Irish unity, that respect is vital."
He added: "Many nationalist voters are willing to exercise their votes interchangeably between the SDLP and Sinn Fein."
Commenting on Tuesday's move, Sinn Fein's John O`Dowd said: "The SDLP leader's action in removing the party whip from Declan O'Loan today exposes that party leadership's identity crisis.
"It would appear from comment by other grassroots SDLP members that there is a wish for democratic debate within that party about its future direction," he said.