Nigel Farage is to step down as leader of the UK Independence Party.
The South East region MEP said he wanted to focus on trying to oust Commons Speaker John Bercow from his Buckingham seat at the next general election.
Mr Farage is expected formally to announce his departure after three years at the party's conference in Southport later.
"I've been the leader of the domestic party for the last three years and frankly doing both of those jobs is too much for any one person - the party is far too big," he told the BBC.
"We're about to embark on a general election campaign in which we're going to have over 500 candidates and because of the way UKIP is structured the leader of the domestic party has got to plan, organise, lead and run that domestic general election campaign.
"And I'm not going to take that burden on because frankly it's too much and anyway I'm going to be busy in Buckingham taking on John Bercow"
By convention, Speakers remain politically neutral and the main parties do not field candidates in their constituencies.
However, Mr Farage said he had decided to stand because MPs had "broken the trust" of the British people, and Mr Bercow "represented the worst" of the Commons.
He is expected to keep his role leading UKIP MEPs in the European Parliament.
The party enjoyed success at the recent European elections, coming second behind the Tories.
