The Electoral Commission says it is investigating whether ex-UKIP donor Arron Banks broke donation rules during the EU referendum.
The probe will look at whether the Leave.EU chairman broke the rules over donations or loans made to campaigners.
It will also look at Better for the Country Ltd, a company of which Mr Banks is a registered director.
Mr Banks said that a judge-led inquiry was needed “to clear this nonsense up once and for all”.
The commission said it launched investigations where there were “reasonable grounds” to believe offences had been committed. It is already carrying out separate probes into the spending returns submitted by the official Remain and Leave campaigns.
Arron Banks investigated over Brexit vote donations
The multimillionaire Brexit donor Arron Banks is to be investigated by the election watchdog over claims he breached campaign finance rules during the EU referendum.
The Electoral Commission will examine whether the insurance tycoon and former Ukip donor broke the law in relation to payments made to campaigners during last year’s referendum.
Better for the Country Limited, a company of which Mr Banks was a director, is at the centre of the investigation. It will examine whether the company, which made donations totalling £2.3 million to five separate registered campaigners during the referendum campaign, was the true source of donations made in its name or whether it acted as an agent, or whether recipients of the money took the requisite legal steps to verify the identity of the firm.