Nigel Farage has accused Ukip leader Gerard Batten of damaging both the party and the Brexit cause by taking far-right activist Tommy Robinson on as an adviser.
Mr Farage said he would write to Ukip’s ruling National Executive Committee to demand a vote of no confidence to remove Mr Batten as leader.
But Mr Batten defended his decision to give Mr Robinson, real name Stephen Yaxley-Lennon, a role advising him on rape gangs and prison reform, describing the English Defence League founder as “courageous”.
The decision was announced shortly after Ukip’s NEC voted not to stage a ballot on whether to allow Mr Robinson to join the party.
Mr Batten denied moving Ukip to the extremes or opening its doors to racists, and said Mr Robinson would help him turn the party into a “mass movement … a party for ordinary people”.
He told BBC Radio 4’s World At One: “I think he is very courageous, he is very brave in what he does.
“He stands up for these victims of industrialised sexual abuse and by and large I think he does positive things.”
“I was always insistent as leader that nobody could join us if they’d been members of the EDL or the British National Party,” Mr Farage told the Press Association.
“I wanted to make tough arguments about illegal immigration, what was happening in the Mediterranean and all the rest of it without any taint of racism or Islamophobia.
“I can’t believe that the current Ukip leader wants to welcome in the founder of the EDL, a man who’s done four prison sentences, lives under a pseudonym and wherever he goes there’s violence.
“The whole thing is crazy. And this at the very moment when Brexit is in a right mess, when both Tories and the Labour Party are breaking their promises to the electorate.
“There’s a massive opportunity for Ukip, a big opportunity for millions to say to the Government Brexit is not going the right way, and all Gerard Batten wants to do is get this, frankly, thug involved with the party.”
Mr Farage said he was concerned a pro-Brexit rally Mr Batten and Mr Robinson are planning for December 9 in London could undermine efforts to defeat Theresa May’s Withdrawal Agreement in a Commons vote in the following days.
“It’s a march through Westminster,” said Mr Farage.
“I know what will happen. It will be full of skinheads and people with tattoos to the eyeballs, they’ll all be drunk before they get there, there’ll be punch-ups and God knows what.
“And the next day, it’s the vote on the Brexit thing.
“And this is how Brexit will look, it’ll look like a bunch of far-right football thugs, and I am absolutely disgusted by it.
“I’ve held my silence as a former party leader until now but we now have to get rid of this bloke as leader because he’s not just damaging Ukip, he’s damaging Brexit.”
Mr Farage did not rule out returning as a candidate for the Ukip leadership himself but said it was “not top of my bucket list” after having already devoted 20 years to the party.
“If it had to be done, I’d consider it,” he told PA.
“This is not taking the party in the wrong direction. He has known me for 26 years, he knows what kind of person I am.”
Challenged over comments Mr Robinson previously made about Muslim immigrants, Mr Batten said: “Lots of people say things in their youth which they then regret and retract from later on.”
He said Mr Robinson’s appointment “signals to a lot of people out there that they can come and support us”.
Asked if racists were welcome in Ukip under his leadership, Mr Batten replied: “No, they are not and they never have been and they never will be.”
He added: “I’m taking the party in a new direction, which is that I want to make it a mass movement.
“I want us to be the party that represents ordinary working people, the unemployed who would like a job if they could get one, and small business owners. I want a party for ordinary people.”