Gordon Brown has dismissed Nigel Farage as not a "man of the people", but "a man of the Paypal".
The former Prime Minister mocked the Brexit Party leader and said that it was time the Electoral Commission and the European Parliament investigated the "finances of Nigel Farage and his party".
He raised the issue after it was revealed that the Brexit Party had a PayPal account for people donating less than £500 but that the party could not confirm where all donations came from. It was also recently revealed that Mr Farage had received £450,000 from Arron Banks, the funder of the Leave EU campaign, while still an MEP and had not declared the money.
Speaking at a Scottish Labour European elections rally, Mr Brown said: "Nigel Farage says this election is about democracy but democracy is undermined if we have undeclared, unreported, untraceable payments being made to the Brexit Party - if we have the potential for under-hand, under the counter payments being made.
"You know the history of this, Leave EU, Nigel Farage's campaign is now criminal investigation, it's under three investigations - by the National Crime Agency, by the Metropolitan Police and by the Information Commissioner.
"Arron Banks, the lead funder of Leave EU, and a friend of Nigel Farage, has been under investigation. He's made contacts with Russia and we don't know where his money comes from yet we found out last week 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 a conflict of interest."
He added: "Now we find the Brexit Party is a private company. It doesn't have members, it has shareholders. The lead shareholder with a controlling interest is Nigel Farage. You pay not to become a member but 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 ringgits and probably to his disdain Euros as well.
"If this election is about trust in democracy the Electoral Commission has the power to tell us if they've had questions answered about where the money is coming from, who is getting the money and whether rules are being broken.
"He's not going to be remembered, as he wants, as a man of the people, he's going to be remembered as a man of the Paypal - because that's where the money comes from.
"The EC says it does investigations during campaigns to see if people are breaking the rules so it's quite possible for them, and I challenge them to do this, to say they are investigating. 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."
On Radio 4 today. Brexit Party chairman Richard Tice said: "We've got a PayPal account for people paying less than £500; above that we apply the appropriate Electoral Commission rules."
Asked if he could confirm whether the party takes cash from foreign citizens, Mr Tice said: "I don't sit in front of the PayPal account all day so I don't know what currencies people are paying in, but, as I understand it, the PayPal takes it in sterling."
The Electoral Commission has said it will go to the Brexit Party's office tomorrow "to review the systems in place". "If there's evidence that the law may have been broken, we will consider that in line with our enforcement policy," a spokesperson said.
Under the rules governing donations, amounts below £500 do not have to be declared, however an official donation of £500 or more must be given by a "permissible donor", who should either be somebody listed on the UK electoral roll or a business registered at Companies House and operating in the UK.
Responding to the Mr Brown's comments, Mr Farage said: "Most of our money has been raised by people giving £25 to become registered supporters. And over 110,000 of them now have done that. And frankly, this smacks of jealousy because the other parties simply can't do this."
In his speech Mr Brown also declared the SNP and Conservative parties were locked in a "Punch and Judy" constitutional show.