The Electoral Commission has defended visiting The Brexit Party’s offices to review how the party receives funding.
Party leader Nigel Farage accused the watchdog of acting “in bad faith” and “interfering in the electoral process”.
But the watchdog said there had been “significant public concern” about the way the party raises funds.
Separately, the European Parliament’s advisory committee will look at whether Mr Farage broke rules by accepting funding from campaigner Arron Banks.
Responding to Mr Farage’s comments, an Electoral Commission spokesperson said there was no evidence of electoral offences, but added: “We want to satisfy ourselves that the party’s systems are robust.”
On Monday, former Labour Prime Minister Gordon Brown attacked The Brexit Party for receiving a large amount of money via what he called “undeclared, untraceable payments”.
However, Mr Farage dismissed that attack as a “smear” and suggested there might be collusion between the Electoral Commission and Mr Brown.
“I’m certain of it – I’m certain the establishment are working together,” he told the BBC.
He said the commission had declared itself “absolutely happy with our system” last week, but had changed position “in an act of bad faith, beautifully timed to coincide with Gordon Brown’s speech”.
The watchdog said in response: “Our regulatory work during this campaign – for the European parliamentary elections – has not deviated from our usual approach.
“Our decision to visit is not related to comments made by the former prime minister.”
The commission said it was “independent and impartial”, and regulated “proportionate to the issue, regardless of a party’s politics”.
Under the rules governing donations to political parties, amounts below £500 do not have to be declared.
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.
At an event in Glasgow on Monday, Mr Brown said there was no way of telling whether donations to The Brexit Party – which can be made through PayPal – come from British or foreign sources, and therefore he suspected the system was being abused.
Defending his party’s practises, Mr Farage said: “We make it very clear, we only want your money if you’re on the UK electoral roll.”
“We’re looking for irregularities, if we see things that have come from overseas, we simply send it back,” he said.
The Brexit Party has updated its website since Mr Brown’s speech to say that those making donations or becoming registered supporters must comply with the permissible donor requirements.
In a separate matter, President of the European Parliament Antonio Tajani has asked its Advisory Committee on the Conduct of Members to consider whether Nigel Farage broke parliamentary rules by accepting £450,000 from Leave.EU founder Arron Banks while still an MEP.
Following a Channel 4 investigation into the £450,000, Mr Farage said he did not declare it to the European Parliament because he was about to leave politics and had been seeking a new life in the US.
The earliest the committee – made up of five MEPs – can meet is 4 June. It will examine the circumstances of the case and can hear from the person concerned, before making recommendations to the President.
MEPs found to have acted improperly can be reprimanded, their parliamentary allowance can be withheld, or they can be banned from some activities.
Mr Farage confirmed that he was not talking to Channel 4 News, describing them as “political activists”, and said he would not allow the broadcaster to attend Brexit Party events.
The editor of Channel 4 News, Ben de Pear, said on Twitter he hoped “to resolve our access ban… ASAP”.