Anna Turley, Chair of the Labour Party, wrote to the FCA, urging an investigation into whether Mr Farage is using his public role to benefit those who fund him.
Mr Harbone, a Thailand-based crypto investor, has donated millions to Mr Farage and Reform UK, including a record £9m last August and a further £3m this year.
The letter highlights Mr Farage’s public praise for stablecoins like Tether and his arguments against measures that could limit their growth, including during a private meeting with Bank of England Governor Andrew Bailey.
An ongoing investigation by Westminster’s standards watchdog is also looking into whether Mr Farage broke Commons rules by not declaring a £5m “gift” from the crypto entrepreneur after his election in 2024.