Gordon Brown has 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 that democracy would continue to be undermined if payments to the party were not declared.
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 Electoral 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 accused the former premier of an “absolutely disgusting smear” against his party.
“This from the man who was part of a Labour Party who, through Lord Levy, were making a lot of big donors members of the House of Lords,” Mr Farage said on a campaign visit to Exeter.
“How dare he? Most of our money has been raised by people giving £25 to become registered supporters and nearly 110,000 of them now have done that.
“Frankly, this smacks of jealousy because the other parties simply can’t do this.
“How open can we be? What you have got here are the conspiracy theorists doing their utmost to try and delegitimise what is the fastest-growing political movement this country has ever seen.”
A spokesman for the Electoral Commission 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.
“These rules are in place to ensure fairness and transparency of all political party finance.
“As part of our active oversight and regulation of these rules, we talk regularly to parties, including the Brexit Party, about ensuring they have robust systems in place so that they comply with the law.
“If we see evidence to suggest the rules have been broken, we will consider it in line with our enforcement policy.”