Ex-UKIP leader Paul Nuttall has become the latest senior figure to quit the party over Tommy Robinson's appointment as an adviser.
Mr Nuttall, who sits in the European Parliament as an MEP for North West England, claimed UKIP is "being taken in a direction which I believe is harmful to Brexit".
He criticised the party's association with far-right activist Robinson, who has been installed as a personal adviser to current UKIP leader Gerard Batten.
Mr Nuttall revealed he had taken the decision to quit UKIP after 14 years "with an immense amount of reluctance and regret" and after a period of "much soul-searching".
He said in a statement: "I am resigning because the party is being taken in a direction which I believe is harmful to Brexit.
"The association with Tommy Robinson will simply appal many moderate Brexit voters and inevitably be detrimental to the cause."
He added: "Putting Tommy Robinson front and centre, whilst Brexit is in the process of being betrayed is, in my view, a catastrophic error.
"To conflate Brexit and Robinson at this crucial moment is to put the eurosceptic cause in danger and I cannot and will not be party to that."
Mr Nuttall vowed to continue to serve as an MEP until his term expires next year.
Robinson, real name Stephen Yaxley-Lennon, is the former founder of the English Defence League.
The ex-convict was last month appointed as Mr Batten's adviser on rape gangs and prison reform, despite UKIP's ruling body deferring a decision on whether to allow him to become a party member until after the UK leaves the EU.
Mr Batten, who took over as UKIP leader in February, has insisted the party under his leadership is "not anti-Muslim", but has reiterated his belief that Islam is a "death cult".
Mr Nuttall was UKIP leader between November 2016 and June 2017, but resigned after the party won less than 600,000 votes at last year's general election.
His spell in charge of the party was marked by a row over false claims on his website, including that he lost close friends at the Hillsborough tragedy.
Mr Nuttall campaigned to ban the burka during his time in charge of UKIP.
He follows both fellow former leader Nigel Farage and a huge swathe of UKIP MEPs in quitting the party since the Brexit vote.
Mr Farage has accused Mr Batten of being "obsessed" with both Robinson and Islam, but failed in his attempts to force a no confidence vote in Mr Batten before his resignation.
Scottish MEP David Coburn was also revealed to have left UKIP on Friday, claiming the party is heading towards an "anti-Islam" platform.
UKIP's disappearing MEPs
Of the 24 UKIP MEPs elected at the last EU elections in 2014, 16 no longer represent the party in the European Parliament.
Some 15 of those have left the party altogether.
1. Patrick O'Flynn – QUIT
2. Stuart Agnew
3. Tim Aker – QUIT
4. Roger Helmer – RESIGNED AS MEP (but replaced by Jonathan Bullock)
5. Margot Parker
6. Gerard Batten
7. Jonathan Arnott – QUIT
8. Paul Nuttall – QUIT
9. Louise Bours – QUIT
10. Steven Woolfe – QUIT
11. David Coburn – QUIT
12. Nigel Farage – QUIT
13. Janice Atkinson – EXPELLED
14. Diane James – QUIT
15. Ray Finch
16. William Dartmouth – QUIT
17. Julia Reid
18. Nathan Gill – QUIT
19. Jill Seymour
20. Jim Carver – QUIT
21. Bill Etheridge – QUIT
22. Jane Collins
23. Amjad Bashir – DEFECTED TO TORIES
24. Mike Hookem