Farage, who quit as party leader in 2016 but stayed on as a card-carrying member, fell out with the current leadership after it appointed a far-right activist and convicted criminal named Tommy Robinson as an adviser last month.
Farage, who has himself been accused of fomenting far-right views, said that at a UKIP conference this year he was confronted by "several angry young men... who all seemed to be obsessed with Islam and Tommy Robinson".
"The party of elections is quickly becoming a party of street activism, with our members being urged to attend marches rather than taking the fight to the ballot box," he wrote.
Brexit has been the 54-year-old former trader's dream ever since leaving the Conservatives in 1992 following the signing of the Maastricht Treaty, which deepened European integration.
Farage has spent nearly three decades undermining European institutions and mocking its leading lights, a mission that culminated with the 2016 referendum to leave the EU.
Voted "Briton of 2014" by The Times, Farage maintained a high profile before the referendum but was kept out of the official pro-Brexit campaign, which feared his brand was too divisive.
The Brexit vote helped vindicate some of Farage's methods, but came at a price. "During the referendum I said I wanted my country back ... now I want my life back," Farage said when he resigned as leader two years ago.
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