The UK Independence Party (UKIP) was founded in 1993 by Alan Sked but only rose to prominence after Nigel Farage took over as leader in 2006. Farage quit the party in November 2016 and formed the Brexit Party earlier this year, which has eclipsed UKIP politically.
UKIP leader Gerard Batten has resigned after the party lost all its seats in the European elections last month.
A leadership contest is set to start and the winner will take over on 10 August.
UKIP voters deserted in droves when its former leader Nigel Farage set up the Brexit Party and recruited people like former Tory minister Ann Widdecombe to stand as candidates during the 23 May election.
Mr Batten promised to step down if he lost his seat at the election.
Party chair Kirstan Herriot said UKIP was “incredibly grateful” for his “dedicated service”.
Mr Batten was heavily criticised by Farage for recruiting former English Defence League leader Tommy Robinson as a UKIP candidate in the North West of England and taking advice from him on some areas of policy.
Robinson — who is not a fully fledged UKIP member because of a ban on former EDL and BNP members — was appointed “grooming gangs adviser” in November.
Mr Batten defended Robinson’s right to come out of prison and resume his life and said he had “walked away from the EDL a long time ago”.
But many pro-Brexit voters, especially former Labour voters, found it easier to bring themselves to vote for the Brexit Party rather than UKIP.