The Electoral Commission has said it will attend the offices of Nigel Farage’s Brexit party to “review its systems” after Gordon Brown urged them to investigate concerns over the legality of the party’s funding.

The former prime minister told a Labour rally in Glasgow the commission had the powers to carry out live investigations during elections and issue interim statements on whether it believed there were unanswered legal questions about party funding.

Brown said there were clear risks that democracy was being damaged if the Brexit party was allowed to accept foreign and untraceable donations via the online payments service PayPal. Political gifts of under £500, whether made via PayPal or another route, do not have to be declared.

Farage “is not going to be remembered, as he wants, as the man of the people. He’s going to be remembered as the man of the PayPal, because that is where the money is coming from,” Brown declared.

Following the speech, the Electoral Commission issued a statement, saying it planned to attend the party’s offices on Tuesday. A spokesperson 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.

Facebook Twitter Pinterest Gordon Brown at an EU election campaign rally. Photograph: Murdo MacLeod/The Guardian

“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 is evidence that the law may have been broken, we will consider that in line with our enforcement policy.”

Brown said the European parliament should be investigating the disclosures last week that Farage had received about £450,000 in financial support from Arron Banks, the Eurosceptic businessman who funded Farage’s Leave.EU Brexit campaign during the 2016 referendum. That could be a clear conflict of interest with Farage’s duties as an MEP, he added.

“The Electoral Commission and the European parliament should now investigate the finances of Nigel Farage and the Brexit party,” he said. “Democracy is undermined. [Farage] says the election is about democracy. Democracy is undermined if we have undeclared, unreported, untraceable payments being made to the Brexit party, if we have the potential for underhand and under-the-counter payments being made.

“[If] this election is about trust in democracy, the Electoral Commission has the power before Thursday to tell us if they have 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.”

Farage accused Brown 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,” Farage said on a campaign visit to Exeter.

Richard Tice, the Brexit party’s chairman and co-founder, insisted on Twitter on Monday morning the allegations of illicit foreign funding via PayPal were unfounded.

He tweeted: “The Brexit party only receives money in sterling. The offer stands to send a BBC journalist to come and look at our PayPal account.”

Richard Tice (@TiceRichard) The Brexit Party only receives money in sterling. The offer stands to send a BBC journalist to come and look at our PayPal account. https://t.co/PR8A8nlPVx

Farage said he was indignant about Brown’s allegations: “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.”

At a rally of 1,000 supporters in Bolton on Monday night, Farage went further, claiming the Brexit party had come under a coordinated attack from Brown and the media.

He said his party last week went to the Electoral Commission to show them the checks and methods used and got “a clean bill of health”. He said it was only after Brown’s speech that the commission said they would visit the Brexit party offices, adding: “They are doing so on the basis of absolutely no evidence at all.”

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.”

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Brown had earlier alleged there was precedent for his questions over the Brexit party’s funding. He said there were already three criminal investigations into Leave.EU, by the National Crime Agency, the Metropolitan police and the information commissioner.

“Arron Banks, the lead funder of Leave.EU and a friend of Nigel Farage, has been under investigation. He has big contacts with Russia,” Brown said. “We don’t know where his money comes from and 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 to avoid any conflict of interest.”

On Monday night, a source at Channel 4 News confirmed its reporters had been banned from Brexit party events since last Thursday following their investigation into Banks’ alleged funding of Farage.

A Brexit party spokesperson said they were unaware of any ban but that “doesn’t mean it can’t have been done”, adding: “Channel 4 are just activists. They just are.”