David Trimble has announced his resignation as leader of the Ulster Unionist party.
The party lost four of its five Westminster seats in this week’s election.
Mr Trimble blamed his party's problems on republicans, saying they had not been behaving properly in recent years. He also accused the British government of indulging the republican movement.
Mr Trimble lost his own seat in Upper Bann to David Simpson of the DUP.
He led the Ulster Unionist for 10 years after succeeding James Molyneaux. The party had 10 MPs in Westminster at the time.
Mr Trimble, who has survived repeated challenges to his leadership in recent years, is 61 years old and married with four children.
The election for a new leader is expected to take place in the autumn.