South Down Assembly member and former Stormont minister Dermot Nesbitt has said he will not seek re-election next March.
The UUP member announced his decision on Tuesday night at a party meeting in the constituency, called to prepare for the next Assembly election.
Mr Nesbitt (59) said he was grateful for his involvement in party politics since his early days as an election agent for former Northern Ireland prime minister Brian Faulkner.
The one-time Queen's University academic and environment minister said it had "been a privilege" to have worked with all Ulster Unionist leaders since the 1970s. "I believe that each leader of the Ulster Unionist Party, in his own time, was and is endeavouring to do his best for Northern Ireland," he said.
Mr Nesbitt was part of David Trimble's negotiating team at the talks which concluded the Belfast Agreement. He paid tribute to his colleagues of the time.
"Politically, we were storming the beaches and taking the 'hits' - in the vanguard of political thinking that was so necessary in order to deliver a better future for all in Northern Ireland," he said.