The electoral watchdog is being urged to investigate how Nigel Farage's Brexit Party is financed amid broader concerns over anonymous donations being used as a "cover for dirty money".

Former prime minister Gordon Brown has written to the Electoral Commission to demand a probe into the insurgent party's funding structures.

The row comes after Mr Farage boasted his newly-formed Brexit Party was receiving more than £100,000 in donations a day, as it soared to first place in the polls ahead of the upcoming European elections.

The Brexit Party allows donations of under £500 by Paypal on its website, prompting concern that it could be vulnerable to foreign influence.

At a speech in Glasgow, Mr Brown was expected to say: “Nigel Farage says this election is about democracy. Democracy is fatally undermined if unexplained, unreported and thus undeclared and perhaps under the counter and underhand campaign finance – from whom and from where we do not know – is being used to influence the very elections that are at the heart of our democratic system.

Farage launches his new Brexit Party: in pictures Show all 10 1 /10 Farage launches his new Brexit Party: in pictures Farage launches his new Brexit Party: in pictures Nigel Farage speaks at the launch of his new Brexit Party's campaign for the European elections Reuters Farage launches his new Brexit Party: in pictures Brexit Party candidate Annunziata Rees-Mogg, sister of Conservative MP Jacob Rees-Mogg, speaks at the launch AFP/Getty Farage launches his new Brexit Party: in pictures A supporter waits for Farage to speak AFP/Getty Farage launches his new Brexit Party: in pictures Supporters wait for Farage to speak AFP/Getty Farage launches his new Brexit Party: in pictures Farage's socks Reuters Farage launches his new Brexit Party: in pictures Farage and prospective candidate Annunziata Rees-Mogg wait at the launch AFP/Getty Farage launches his new Brexit Party: in pictures Supporters listen as Farage speaks AFP/Getty Farage launches his new Brexit Party: in pictures Free T-shirts for all attendees AFP/Getty Farage launches his new Brexit Party: in pictures Posters on the seats for supporters of the Brexit Party AFP/Getty Farage launches his new Brexit Party: in pictures A safety sign is pictured AFP/Getty

“Now Mr Farage heads a new Brexit party, which is making questionable claims about the true source of its funding at a time when the Electoral Commission has warned of the dangers of multiple, small, anonymous donations being a cover for dirty money.”

Brexit Party chairman Richard Tice said allegations of impropriety were "absolutely ridiculous" and accused critics of being "jealous" of the party's success.

He told BBC Radio 4's Today programme: "The reality is that we are the fastest growing political movement.

"We have got over 100,000 supporters who have paid at least £25, we've had two investigative journalists come and look at it.

"The reality is people are just jealous of our success."

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Asked if they took donations in foreign currencies, Mr Tice said there was a Paypal account for people paying less than £500 but he did not "sit in front of the account" all day so he did not know what currencies people were using.

Pressed again, he added: "As I understand that is not illegal. Of course we apply appropriate Electoral Commission law.

"The truth is hundreds of thousands of people want to support us, they back what we are trying to do and trying to achieve and they know we are the only party who can be trusted with democracy and trusted to deliver a proper Brexit."

It comes as Mr Farage's outfit was expected to triumph in the European elections on Thursday, as both the Tories and Labour brace for a drubbing at the polls.

A spokesperson for the Electoral Commission said all registered political parties must comply with laws that "require any donation it accepts of over £500 to be from a permissible source", as well as rules for reporting donations, loans, campaign spending and end-of-year accounts.

The commission said: “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.”