Britain's election watchdog has launched a probe into campaign spending by the pro-Brexit group Leave.EU ahead of last year's historic poll, it said Friday.
The Electoral Commission said that an initial assessment concluded that "there were reasonable grounds to suspect that potential offences under the law may have occurred".
The investigation will focus on whether the group had accepted "impermissible" donations and on whether it had fully completed its spending return.
"Once the investigation is complete, the Commission will decide whether any breaches have occurred and, if so, what further action may be appropriate," it said in a statement.
The Leave.EU campaign group was founded by multi-millionaire Arron Banks in July 2015 and was initially backed by former UK Independence Party (UKIP) leader Nigel Farage but it lost out on official campaign status to the Vote Leave group.
But it continued to play an instrumental role and has remained active since the June 23 vote, campaigning for Britain to exit the European single market and for a strict immigration policy.
Banks denounced the Electoral Commission investigation as "clearly politically motivated" on his Twitter account.
The businessman and political donor announced on Wednesday that he will run for a seat in parliament in a June 8 snap election called in a surprise announcement this week by Prime Minister Theresa May.
He will stand for parliament in Clacton, in the southeast of England.
The constituency was previously held by Douglas Carswell, UKIP's sole MP, who quit the party last month and announced this week that he would not stand again in the June election.
Britain's Lib Dem leader campaigns against 'hard Brexit'
Tim Farron, whose centre-left party holds just nine seats but is hoping to make gains in the surprise election on June 8, said only the Lib Dems could prevent a so-called "hard Brexit", which would involve Britain leaving the European single market when it quits the EU.
He condemned Prime Minister Theresa May's decision to head for a hard Brexit, saying voters should see the election as a "historic opportunity to change the direction" of the country.
Britain remains deeply divided after last June's referendum on whether to quit the EU, when 51.9 percent voted for Brexit.
May made the surprise call for an election this week, hoping to capitalise on bitter infighting in the main opposition Labour party and secure a boosted majority for her Conservative party going into two years of gruelling Brexit negotiations with Brussels.
The Lib Dems too are hoping to pinch votes from Labour, which is split on how to approach Brexit as well as the future of its embattled leader Jeremy Corbyn.
Watch: Opposition begins election campaign
Farron, making his first campaign stop in the northern city of Manchester, said the Conservatives "now assume a coronation" but that they had "every reason to be afraid of the Liberal Democrats".
The Lib Dems were junior partners in a Conservative-led coalition government from 2010 to 2015 but were then crushed in a general election when voters abandoned them due to their broken promise to halt large hikes in university fees.
A YouGov poll released Thursday put the Lib Dems on 12 percent of voting intentions, well behind the Conservatives at 48 percent.
May holds a slim majority of 17 seats in the 650-seat House of Commons and polls suggest she is likely to strengthen her grip on power.
But the premier still warned her party against complacency on Friday.
"I'm not taking anything for granted," she said on the campaign trail in her home constitutency of Maidenhead, southern England.
"The result is not certain."
