Campaign group Leave.EU is under investigation over "potential offences" during last year's Brexit referendum.
The Electoral Commission is looking into the spending returns of the group, saying there were "reasonable grounds" to suspect offences may have occurred.
Leave.EU has faced claims it failed to declare advice given by an American social media strategy firm.
Leave.EU founder Arron Banks said he would "vigorously" contest the allegations.
The group, which was backed by then UKIP leader Nigel Farage, ran a separate campaign to Vote Leave, the officially-designated group backed by senior Tories.
The Guardian reported last month that Leave.EU was advised by Cambridge Analytica, a company hired by Donald Trump during his US election campaign, which uses artificial intelligence to personalise political messages according to the things voters say and "like" on Facebook.
It said campaign filings published by the watchdog in February made no reference to Leave.EU's association with the Washington DC-based company, which also has offices in London and New York. It said the issue had been brought to the watchdog's attention by Labour MP Stephen Kinnock.
Cambridge Analytica is largely owned by Robert Mercer, a US businessman who helped to fund Donald Trump's presidential campaign.
The Electoral Commission said the investigation would focus on whether one or more donations - including of services - accepted by Leave.EU was "impermissible" under UK electoral law and, as a result, whether its spending return was complete. It would not elaborate further.
Under UK law, foreign citizens cannot donate to political parties or other organisations unless they are on the electoral register.
Companies and other corporate donors - including limited liability partnerships and unincorporated associations - have to be based and registered in the UK.
"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 watchdog said.
Mr Banks, a leading UKIP donor in recent years, tweeted that the watchdog had "allowed the government to spend £11m on a pack of remain lies" - a reference to a controversial pamphlet backing EU membership sent to every household in the country.
The insurance millionaire, who is a close ally of Nigel Farage and is considering standing in the general election in Clacton, told the Observer last month that Brexit had been a "war" and he "did not give a monkey's" about the Electoral Commission.
Andy Wigmore, Leave.EU's director of communications, said the group had done "nothing wrong".
He suggested there may a link between the announcement and Mr Banks' interest in the Essex seat, tweeting: "Yawn. Our monthly investigation by the Electoral Commission. How convenient just before Arron Banks stands to become an MP."
Registered campaigners were required to declare what donations and loans they had received of more than £7,500 between 1 February and 23 June 2016. Details were published in February.
The watchdog is already investigating the spending returns of Vote Leave and the official Remain campaign Stronger In, although it says it is too early to say whether offences have been committed in these cases.