Ukip deputy leader Mike Hookem has quit and will run for party leader.
He said he can no longer support the direction of party leader Gerard Batten and wants to offer “a real alternative” to his leadership that would “consolidate and rebuild our party”.
Mr Batten has already said he will stand down on June 2, launching a leadership contest.
In a letter sent to party officials at the close of polls in the European elections, Mr Hookem said: “I believe Ukip always has been and always should be a libertarian party that encourages and promotes common-sense policies with a broad electoral appeal.
“However, under Mr Batten’s leadership, and despite my appeals, Ukip has been derailed from this objective.
“Mr Batten’s policy direction and associations have given the mainstream media the ammunition to label our party ‘extreme’ and ‘far-right’, accusations I do not believe to be true.”
Mr Batten has been Ukip leader for 16 months and appointed former English Defence League leader Tommy Robinson as his political adviser in November 2018.
Mr Hookem, an MEP who stood for re-election this week, said Brexit had been a “golden opportunity for Ukip”, which was led by Nigel Farage before he stepped down in 2016.
He said Mr Batten had “squandered our natural advantage and seriously undermined 26 years of work in the process”.
Ukip languished in the polls ahead of the European elections, in stark contrast to the Brexit Party’s surge in support which saw Mr Farage’s new group overtake every other party.
Revealing his growing frustration, Mr Hookem said: “I have been desperate to act to stop the increasing isolation and marginalisation of our party in recent months.
“However unlike some who found alternative parties, I stayed loyal to the membership and delayed my resignation to minimise any potential electoral impact.
“I also wanted to make sure my resignation was unconditional on the outcome of yesterday’s European elections.”
Why are you making commenting on The National only available to subscribers?
We know there are thousands of National readers who want to debate, argue and go back and forth in the comments section of our stories. We’ve got the most informed readers in Scotland, asking each other the big questions about the future of our country.
Unfortunately, though, these important debates are being spoiled by a vocal minority of trolls who aren’t really interested in the issues, try to derail the conversations, register under fake names, and post vile abuse.
So that’s why we’ve decided to make the ability to comment only available to our paying subscribers. That way, all the trolls who post abuse on our website will have to pay if they want to join the debate – and risk a permanent ban from the account that they subscribe with.
The conversation will go back to what it should be about – people who care passionately about the issues, but disagree constructively on what we should do about them. Let’s get that debate started!
Callum Baird, Editor of The National
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules here