The Electoral Commission has opened an investigation into Leave donor Arron Banks.
The probe will look into whether Banks breached spending rules during the EU referendum campaign.
Banks told Sky News that the decision was politically motivated.
Read more: Donor Arron Banks cuts ties with Ukip
In October, the Electoral Commission fined a number of groups for breaking campaign finance rules during the Brexit campaign.
It fined the European Movement of the UK Limited £8,750 for delivering "an incomplete and inaccurate" spending return. Ukip also faced a fine of £1,000 for delivering a donation and loans report late.
Five campaigners registered donations from Better for the Country Limited, of which Banks is a director, the Electoral Commission said. The donations totalled £2.35m.
Bob Posner of the Electoral Commission said: “Interest in the funding of the EU referendum campaigns remains widespread.
"Questions over the legitimacy of funding provided to campaigners at the referendum risks causing harm to voters’ confidence. It is therefore in the public interest that the Electoral Commission seeks to ascertain whether or not impermissible donations were given to referendum campaigners and if any other related offences have taken place.”