Zia Yusuf has resigned as the chairman of Reform UK, saying working to get the party elected was no longer "a good use of my time".
In a post on social media, Yusuf did not expand further on his reasons for stepping down.
However, it comes after he said it was "dumb" for the party's newest MP to call on Sir Keir Starmer to ban the burka.
Reform leader Nigel Farage said he was "genuinely sorry" Yusuf was resigning, describing him as "enormously talented".
Farage said Yusuf was "a huge factor" in the party's success in last month's elections, when Reform won a by-election, two mayoral races and gained 677 new councillors.
However, he told GB News he believed Yusuf had "had enough" of politics, which can be "totally unrelenting".
Farage said he had "suspicions" Yusuf might quit after he seemed "very disengaged" when the pair spoke on Wednesday morning but was only given a "10-minute warning" his resignation was coming.
Asked about reports that some in the party found Yusuf difficult to deal with, Farage said "not everyone got on with him".
He added: "Were his interpersonal skills at the top of his list of attributes? No. But I always found him, with me, very polite."
