The Electoral Commission has launched a review into the Brexit Party’s financial systems and will visit the party’s headquarters on Tuesday.
The move comes after former prime minister Gordon Brown called for an investigation to be carried out into the finances of Nigel Farage and the Brexit Party.
Mr Brown, speaking in Glasgow on Monday, said democracy would continue to be undermined if payments to the party were not declared.
A commission spokesman said: “The Brexit Party, like all registered political parties, has to comply with laws that require any donation it accepts of over £500 to be from a permissible source.
“It is also subject to rules for reporting donations, loans, campaign spending and end of year accounts. We have already been talking to the party about these issues.
“As part of our active oversight and regulation of these rules, we are attending the Brexit Party’s office tomorrow to conduct a review of the systems it has in place to receive funds, including donations over £500 that have to be from the UK only.
“If there’s evidence that the law may have been broken, we will consider that in line with our Enforcement Policy.”
And the former prime minister challenged the Electoral Commission and the European Parliament to indicate whether they are investigating the party, or say whether questions over dubious payments had been answered, ahead of the European elections on Thursday.
Mr Brown said: “The Electoral Commission and the European Parliament should now investigate the finances of Nigel Farage and the Brexit Party.
“Democracy is undermined when we have undeclared, unreported, untraceable payments being made to the Brexit Party.
“We have the potential for underhand and under-the-counter payments being made.
“You know the history of this – Leave.EU, Nigel Farage and Arron Banks’ campaign is now under criminal investigation.
“There’s three investigations – one by the National Crime Agency, one by the Met Police and one by the Information Commissioner.
“Arron Banks, the lead funder of Leave.EU and the friend of Nigel Farage has been under investigation – he has made contacts with Russia. We don’t know where his money comes from.
“And yet we find out last week that he has given £450,000 in payments to support Nigel Farage, while Nigel Farage was in a public office in the European Parliament, who should have been declaring the payments that he was receiving from anyone to avoid any conflicts of interest.”
Mr Brown raised concerns over the method of payment used to process donations by supporters of the party.
He said: “Now we find the Brexit Party that has been formed is not a party, it’s actually a private company. It doesn’t have members, it has shareholders.
“The key shareholder, with the controlling interest, is Nigel Farage. And you pay money not to become a member, but to become a supporter, and you pay through PayPal and you cannot discover whether the money is coming from foreign sources or British sources.
“Indeed, you can pay to this party in Russian roubles, American dollars, Malaysian ringgit and, of course, probably to his disdain, euros as well, to join that Brexit Party.”
Mr Brown added: “If this election is about trust in democracy, the Electoral Commission has the power before Thursday to tell us if they’ve had questions answered about where the money is coming from, who is giving the money, whether the money is coming from foreign sources, including America and Russia, and whether rules are being broken.
“This is a not a party in the ordinary sense, this is a private company. It’s run by Nigel Farage, donations go through PayPal, and he’s not going to be remembered as he wants, as the man of the people. He’ll be remembered as the man of the PayPal, because that’s where the money’s coming from.”
Mr Brown continued by challenging the commission to indicate before Thursday whether they are investigating, or if questions on the “dubious issues” around such transactions had been answered.
He said: “Democracy is ill-served and trust in democracy will continue to be undermined if we have no answers to where the money is coming from and why these declarations to the European Parliament have never been made.”
Mr Farage told a rally of 1,000 supporters in Bolton on Monday evening the Brexit Party had come under a co-ordinated attack from Mr Brown and the media for alleged financial impropriety.
He said his party last week went to the Electoral Commission to show them the checks and methods used to ensure they are in line with electoral law, adding: “They gave us a clean bill of health.”
Mr Farage said it was only after Mr Brown’s speech the commission said they would visit the Brexit Party offices.
He added: “They are doing so on the basis of absolutely no evidence at all. They are doing so in an act of bad faith against what they told us to our faces last week.
“They are directly, politically interfering in a national election in this country. It’s a disgrace.”
Mr Farage said the board of the commission were all Remain supporters and it, along with the two party system, the House of Lords and the voting system, needed to be “looked at.”