Campaign group Leave.EU is under investigation over "potential offences" relating to last year's EU referendum, according to the Electoral Commission.
The watchdog is looking into spending returns of the group, after an assessment concluded "there were reasonable grounds to suspect that potential offences under the law may have occurred". The campaign group's boss said she was "flabbergasted" at the investigation.
An Electoral Commission spokesperson 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.
The time taken to complete an investigation varies on a case-by-case basis. 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.
The group, was founded by Arron Banks and backed by former Ukip leader Nigel Farage. It ran a separate campaign to encourage Brits to vote for Brexit against the officially-designated group Vote Leave, which had been backed by senior Tories.
Leave.EU's chief executive Liz Bilney said she was "flabbergasted" at the investigation.
"The Leave.EU electoral return was signed off by an independent auditor, we consulted with legal opinion at every stage and employed a compliance officer to ensure EC guidelines were followed to the letter," Bilney said.
I am therefore flabbergasted to learn that we are now under further investigation.
We fully intend to counter the Commission's excessive measures with our own inquiry into the level of attention given to other EU referendum campaigns.
We wish to bring to account the Commission's numerous lapses in professionalism and fair treatment.
It is deeply worrying to witness a regulator operating at the heart of our democracy and with such brazen disregard for rules, regulations and equitable treatment.
Farage tweeted on the timing of the EC's announcement:
The watchdog is also investigating the spending returns of Vote Leave and the official Remain campaign Stronger In, though it has said it's currently too early to say whether any offences have been committed in those instances.