Zia Yusuf has resigned as chairman of Reform UK, saying working to get the party elected was no longer "a good use of time".
Party leader Nigel Farage described Yusuf as a huge factor in Reform's success in last month's elections, but added that he believed he had "had enough" of politics.
It comes after Yusuf said it was "dumb" for Reform's newest MP to call on Sir Keir Starmer to ban the burka - a veil worn by some Muslim women that covers the face and body.
In a post on X, Yusuf wrote: "11 months ago I became chairman of Reform. I've worked full time as a volunteer to take the party from 14 to 30% [in national polls], quadrupled its membership and delivered historic electoral results.
"I no longer believe working to get a Reform government elected is a good use of my time, and hereby resign the office."
Farage said he had "suspicions" Yusuf might quit after he seemed "very disengaged" when the pair last spoke.
Shortly after his resignation, Nigel Farage appeared on GB News saying: “Not everyone got on with him. You know what? It’s called politics.”
He added: "Were his interpersonal skills at the top of his list of attributes? No. But I always found him, with me, very polite."