Sinn Fein political adviser, Breandan MacCionnaith, has resigned from the party.
Breandan MacCionnaith has left Sinn Fein
Mr MacCionnaith had been adviser to the party's Upper Bann assembly member John O'Dowd, who said he had left the party two weeks ago.
He rose to prominence as spokesman for Garvaghy Road residents over the Drumcree parade dispute and was an independent nationalist councillor.
He has not yet spoken about his decision to leave Sinn Fein.
Mr O'Dowd confirmed the resignation of Mr MacCionnaith as both his adviser and as a party member, but said he remained a "friend and comrade".
He declined to say what was behind the surprise departure.
"If Breandan wants to give any more details that is a matter for him," he said.