The SDLP's Tim Attwood has announced he will not stand for re-election to Belfast City Council.
The Black Mountain representative is his party's longest serving member on the council.
His decision comes after a vote by his party's membership to form a political partnership with Fianna Fáil.
Mr Attwood said he was "not an advocate" of the partnership, but respected "the outcome of the vote and the will of the SDLP members".
He added that although he would not contest further elections, he would remain a member of the party.
Mr Attwood's decision follows a move by his party colleague, Claire Hanna, who resigned her position as the SDLP's Brexit spokeswoman in opposition to the Fianna Fáil deal.
Mr Attwood is currently the leader of the SDLP group on the council.
First elected in 1995, Mr Attwood said he had decided not to contest May's election for "personal and political reasons".
"I have the greatest respect for the leadership of the SDLP and recognise that in a post-conflict situation, there is a natural and wider debate about political realignment on this island," said Mr Attwood.
"The reality is, my politics remains steadfastly rooted in the social democratic and labour tradition embodied by the SDLP and parties of the Party of European Socialists."
In his statement, Mr Attwood also paid tribute to former party colleagues, including "John Hume, Seamus Mallon, Brid Rodgers and so many others".
"They are examples of real heroism in our time," he said.
"When it was not easy or fashionable to preach the message of politics and peaceful constitutional consensual politics, the SDLP never wavered."
During his time on Belfast City Council, Mr Attwood has been vocal on a number of issues, including the regulation of bonfires.
In June 2018, Mr Attwood pulled down an IRA sign close to the peace line corner of Falls Road and Northumerland Street, and called for reciprocal action to be taken in loyalist areas.
He previously served as his party's director of development.