Ukip MEP and former leader Paul Nuttall has quit the party in protest at English Defence League (EDL) founder Tommy Robinson being appointed as an adviser.
Mr Nuttall labelled the move a "catastrophic error", as he became the latest in a wave of prominent figures to leave Ukip complaining of the direction it has taken under leader Gerard Batten.
Mr Batten has drawn criticism for appointing activist and former EDL leader Mr Robinson as an adviser on rape gangs and prison reform.
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 said he felt the party was being taken in in a direction he believed to be "harmful to Brexit".
“The association with Tommy Robinson will simply appall many moderate Brexit voters and inevitably be detrimental to the cause," he said.
“The Tommy Robinson issue should have been shelved and debated within the party following Brexit, in line with the sensible decision taken by the National Executive Committee last month.
“The party leadership and my MEP colleagues have been aware of my views on this issue for some time.
“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.”
The move came as Ukip’s leader in Scotland, MEP David Coburn, also resigned in protest at the “English nationalist direction” adopted by the party.
Mr Coburn also accused the party of promoting anti-Islamic policies.
Writing in a letter to the Ukip party chairman and the chair of the party in Scotland, Mr Coburn said: “As a unionist, I abhor English nationalism as much as I abhor Scottish nationalism.
“The party has been infiltrated by people with an alternative agenda, which is not the one on which I stood when I was elected and sadly does not represent the values for which Ukip once stood.
“I did not run on an anti-Islam platform. Unfortunately, this seems to be the direction that Ukip is taking – obsessing about this issue to the exclusion of all else at a time when we might lose the Brexit we fought so hard for.”
The latest resignations mean more than half of the 24 Ukip MEPs elected in 2014 no longer represent the party.
Former London mayoral candidate and serving London Assembly member Peter Whittle also announced he was quitting Ukip, while the party's most high-profile figure, Nigel Farage, quit earlier this week after calling for Mr Batten to be ousted over the appointment of Mr Robinson as an adviser.
In response to Mr Farage’s criticism, Mr Batten quit the Europe of Freedom and Direct Democracy grouping in the European Parliament on Friday, slamming "continual attacks on me and Ukip in the UK media."
Ukip sources said that the rest of the party’s MEPs currently remain in the EFDD.
But MEP Jill Seymour announced she was quitting her role as the party’s transport spokeswoman, saying she was “struggling to be loyal to a party that is changing direction”.
Ms Seymour said the leader’s decision to “stand by controversial figures” had caused unrest throughout the party and she had “begged” him to change track.
In a letter to Mr Batten, she told him: “You have made it difficult for me, and many, to stay true to the party and our leader, so I feel I have no alternative but to resign from your front bench team, and consider my future.
“I appeal to you to reconsider your present direction; to focus purely on fighting the establishment who are clearly betraying millions of us that wish to leave the EU. This is the only subject this country should be discussing right now.”