Veteran Alliance Party MLA Trevor Lunn has quit the party, saying that an "irreconcilable internal problem" was the reason for his sudden departure.
Mr Lunn will remain as an independent MLA in Lagan Valley and said he did not want to "get into a slanging match" following his resignation.
The news was confirmed by the Alliance Party, which announced "Trevor Lunn's resignation from the party and intention to remain as an independent MLA".
A spokesperson for the party said: "We are sad to learn of Trevor's resignation.
"Party leader Naomi Long MLA met Mr Lunn, but the reasons for his resignation remain unclear.
"He had said for some time he intended to stand down from politics at the next election.
"It is sad he has chosen to end his political career outside the Alliance family.
"We recognise his contribution to the party. We wish him well for the future."
The MLA has since expressed surprise at the timing of the Alliance statement, but confirmed that he had resigned from the party and would be remaining as an independent MLA.
He said: "I'm a bit surprised that they've released it quite so quickly."
But he added that the party was "wrong" to say that the reasons for his resignation were "unclear".
Mr Lunn, who has been an MLA for almost 13 years, said: "The party can say what it likes," but added: "I don't want to get into a slanging match with them."
Mr Lunn said there had been an "irreconcilable internal problem".
When asked if there was any dispute on policy or if it was other issues such as constituency or personality clashes, he said there was no policy dispute.
Mr Lunn (73) has been MLA for Lagan Valley since 2007 and is a former Mayor of Lisburn.
Late last year, he apologised after video footage was posted on social media appearing to show him slapping a dog while on a visit to a dog walking club.
Replying to an angry response on social media, he said he was an animal lover and would be mortified if people thought he "struck a dog deliberately".
He previously made headlines for backing gay marriage after having voiced his opposition for a period of time.
In July 2018, he paid tribute to the NHS after a stay in hospital.
Mr Lunn took over the Alliance candidacy for Lagan Valley from Seamus Close, the long-time representative for the area.
At the time it was predicted that Alliance would struggle to hold on to the seat, but Mr Lunn's performance saw the new candidate elected fourth out of the six seats available.
In the 2017 Assembly election, Mr Lunn received over 6,000 first preference votes, the third highest in Lagan Valley.