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.
“As I said just last week, he was a huge factor in our success on May 1st and is an enormously talented person,” he wrote on social media.
“Politics can be a highly pressured and difficult game and Zia has clearly had enough. He is a loss to us and public life.”
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.”
A former banker who sold his tech start-up company for more than £200m, Yusuf has described himself as a “proud British Muslim patriot”.
He became Reform UK’s chairman shortly after last year’s general election, after previously being a member of the Conservative Party.
He revealed that he had donated £200,000 to the party during the campaign.
As chairman, he was given the job of professionalising the party, wooing donors and increasing Reform UK’s activist base.