The Leave.EU campaign group is facing an official investigation into "potential offences under the law" over its European Union referendum spending returns.
The Electoral Commission - the official elections watchdog - said the move followed an assessment there were "reasonable grounds" to suspect the law had been broken.
The announcement drew a furious response from Leave.EU's chairman - the millionaire former Ukip backer Arron Banks - who threatened legal action against the commission.
In a statement, the commission said the inquiry would be investigating whether the group had received "impermissible" donations, including in the form of services.
"This followed an assessment which concluded that there were reasonable grounds to suspect that potential offences under the law may have occurred," it said.
"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.
"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."
Mr Banks, who has said he intends to stand as a candidate in the general election, claimed the investigation was "politically motivated" and that the group would not co-operate with the commission's inquiry.
"We fully and correctly complied with the Electoral Commission rules on spending during the referendum. We have answered all of their questions," he said in a statement.
"Today's announcement is politically motivated and the timing is intended to cause maximum damage just before the general election.
"We will not be co-operating any further with the commission and we will see them in court."
Mr Banks, a close ally of former Ukip leader Nigel Farage, set up Leave.EU in the hope of gaining the designation as the official Leave campaign in last year's referendum.
However, they lost out to the Vote Leave group spearheaded by Boris Johnson and other prominent Conservatives.
The Electoral Commission has previously announced that 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 involved in the campaign.
Leave.EU declared spending of £693,022 - close to the legal limit of £700,000 for the non-official campaign groups.
Mr Banks has said he intends to stand in the election in Clacton, the seat held by Ukip's only MP, Douglas Carswell, until he quit the party to sit as an independent. Mr Carswell, a former Tory, has said he will not be defending the seat.