Nigel Farage has quit UKIP, saying the party's leader Gerard Batten seems to be obsessed with Islam and ex-English Defence League leader Tommy Robinson.
Speaking on LBC radio, the former leader of the Eurosceptic party said he made the decision with a "heavy heart".
But he said he did not "recognise" the party anymore and it was turning a blind eye to extremist politics.
Mr Batten survived a vote of no confidence on Monday, after he appointed Mr Robinson as an adviser.
However, the party's ruling body said it did not endorse Mr Robinson's appointment and he was barred from membership "through his associations".
Mr Farage - who joined the party's forerunner, the Anti-Federalist League, in 1992 - said he had worked "tirelessly" for UKIP with the aim of forcing a vote to "get us back our independence" from the EU.
But he said under Mr Batten's leadership "a lot has changed".
"[Mr Batten] seems to be pretty obsessed with the issue of Islam, not just Islamic extremism, but Islam, and UKIP wasn't founded to be a party fighting a religious crusade," he said.
"[He is] also obsessed with this figure Tommy Robinson..."
Mr Farage said the association of the former EDL leader brought "scuffles" and "violence" to the party, adding: "Many have criminal records, some pretty serious, and all of it has been dragging UKIP away from being an electoral party into a party of street activism."
Mr Batten and Mr Robinson, whose real name is Stephen Yaxley-Lennon, are due to appear together at a pro-Brexit march this weekend. Mr Farage said the "image" that will give Brexit is "something that our enemies will use against us for perhaps many years to come".
"Damaging UKIP is one thing, damaging the Brexit cause is even worse," he added. "The brand has now been so damaged, so tarnished, that it is not able to pick up and won't be able to pick up the political opportunity that is there staring it in the face."
'Committed to a true Brexit'
After Mr Farage quit the party, Mr Batten said he gave "full credit" to him for the part he played in winning the 2016 EU referendum for the Leave side, but said Mr Farage had "left the party in spirit" after the vote.
Mr Batten also said: "[UKIP] stood on the edge of destruction in February 2018 and Nigel chose to play no part in rescuing it. I am pleased that I was able to do precisely that.
"Under my leadership, the party has been saved financially, recruited thousands of more members, and we have risen in the polls. We are going from strength to strength."
He added: "I am not going to return like with like in this instance and instead wish Nigel well in his media career. I doubt he will be asking me to go on his show."