Nigel Farage to be examined over £450,000 payment from Arron Banks
The European Parliament's advisory committee will look at whether Mr Farage broke rules by accepting funding from Leave campaigner Arron Banks.
Nigel Farage said he did not declare the £450,000 sum to the European Parliament because he was about to leave politics and had been seeking a new life in the US.
The committee will examine the case before advising the European Parliament President Antonio Tajani.
The committee can meet on 4 June.
MEPs found to have acted improperly can be reprimanded, their parliamentary allowance can be withheld or they can be banned from some activities.
The payments from Arron Banks to Nigel Farage were revealed by a Channel 4 News investigation.
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".
Electoral Commission visit
Separately, the Electoral Commission has defended visiting The Brexit Party's offices to review the party's online fundraising activities.
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.
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."
The Brexit Party said it was "pleased, but not surprised" by the commission's announcement that it had not seen evidence of electoral offences.
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.