NI21's only elected councillor has quit the party.
Johnny McCarthy's move is the latest blow to the troubled political venture led by former Ulster Unionist Basil McCrea.
Co-founder and fellow UUP defector John McCallister has already left the party, as has its former chair and one-time European election candidate Tina McKenzie.
Mr McCarthy, who was elected to Lisburn and Castlereagh council last year, announced his departure on social media.
In a Facebook post, he said: "Serious post for once.
"After a long period of consideration I have decided to leave NI21.
"I feel I can work better outside of the party.
"Some people may wonder why I have stayed as long as I did and it is because I fully believed in the idea of NI21 and the need for a party that is non-sectarian and promotes the use of EVIDENCE to create policy and not ideology.
"I do regret having to come to this decision but it is the right thing for me to do."
In a light-hearted sign off, he added: "Now has anyone got the DUP's number?"