DUP councillor Guy Spence has resigned from the party because he has "lost all confidence" in it.
Mr Spence, who served as deputy mayor of Belfast, said he had resigned on Friday and would retire from Belfast City Council at the end of March.
He said he had watched "party values and integrity be tossed aside often by personal agendas for individual gain" over the past year.
The party thanked Mr Spence for his "service as a councillor."
"We will be co-opting his replacement in due course," a spokesman added.
In a statement on Twitter, Mr Spence said: "My resignation and retirement has been accelerated by recent events including a blatant attempt to cause me harm - something I'm not prepared to tolerate".
Mr Spence said he had submitted a letter to the Democratic Unionist Party and written to the chief executive of Belfast City Council.
Yesterday I resigned from the DUP & at the end of this month I will retire from Belfast City Council.— Guy Spence (@GuySpence) March 7, 2020
It has been a privilege to serve the people of my home District of Castle over the last nine years.
Thanks to everyone who has supported me throughout my time in Council! pic.twitter.com/kvrAWRvHjk
Mr Spence represented the Castle district and until the end of the month will be an independent member of the council.
He said he planned to step back from politics and pursue a new career.
"It has been a privilege to serve the people of my home District of Castle over the last nine years," he said.
"Thanks to everyone who has supported me throughout my time in Council," he added.