One of the two high-profile pro-Brexit campaign groups in the EU referendum is being investigated, amid suspicions of "potential offences".
The Electoral Commission has said it is looking into whether one or more donations accepted by Leave.EU was "impermissible" and if its spending return was complete.
The inquiry by the official elections watchdog comes after it found "reasonable grounds to suspect that potential offences under the law may have occurred".
Leave.EU was co-founded by millionaire businessman and former UKIP donor Arron Banks, and was backed by the party's then leader Nigel Farage.
But it was not designated as the official Leave campaign in last June's referendum.
That role was given to Vote Leave which had the support of senior Conservatives Boris Johnson and Michael Gove.
The commission has previously announced it is investigating the spending returns of both Vote Leave and the official Remain campaign, Britain Stronger In Europe, as well as a number of other groups.
Leave.EU declared spending of £693,022 - close to the legal limit of £700,000 for the non-official campaign groups.
On Twitter, Mr Banks said: "The Electoral Commission allowed the Government to spend £11m on a pack of remain lies.
"We will be vigorously defending their allegations."
He added: "It's clearly politically motivated just as the Clacton campaign is announced."
Mr Banks said this week he will be standing as a candidate for the Essex seat in this June's General Election.
A commission statement said: "The Electoral Commission has begun an investigation into Leave.EU's EU Referendum spending return.
"This followed an assessment which concluded that there were reasonable grounds to suspect that potential offences under the law may have occurred.
"The investigation is focused on whether one or more donations - including of services - accepted by Leave.EU was impermissible; and whether Leave.EU's spending return was complete."
It added: "Once the investigation is complete, the commission will decide whether any breaches have occurred and, if so, what further action may be appropriate, in line with its enforcement policy."