The European Parliament has formally opened an investigation into claims that Nigel Farage failed to declare nearly half a million pounds in gifts from an insurance tycoon under investigation by the National Crime Agency.

A source close to European Parliament president Antonio Tajani confirmed to The Independent that the Brexit Party leader’s case had been referred to the advisory committee that scrutinises financial matters.

It comes as the UK’s own Electoral Commission visited the party’s offices on Tuesday as part of a probe into its finances.

It was claimed last week that the Brexit Party leader has been given as much as £450,000 in kind by Arron Banks, including a chauffeur-driven car, rent and bills on a £4.4m Chelsea home, and lavish trips to the United States to meet with right-wing politicians.

But none of the gifts, detailed in invoices seen by Channel 4 News, were declared on Mr Farage’s register of interests with the European parliament, which is designed to prevent MEPs from keeping their conflicts of interests secret.

The Independent revealed at the weekend that one of the legislature’s quaestors – senior MEPs responsible for the body’s financial and administrative matters – had called on Mr Tajani to “investigate these apparent contraventions as a matter of urgency”.

The launch of the inquiry comes days before British voters are set to go to the polls for the European parliament elections, with Mr Farage’s Brexit Party thought to be heading for a runaway victory with an anti-EU, anti-establishment message.

The quaestor, Catherine Bearder, a Lib Dem MEP, had cited parliamentary rules governing conflicts of interests, including Article 6(1) covering the renumerated attendance of events.

Under European parliament rules, Mr Farage could be fined up to around €10,000 (£8,800) through withholding of his subsistence allowance if he is found to have broken the rules. He could also be suspended from “all or some of the activities of parliament for a period of between two and 30 days” – though his right to vote cannot be removed.

Responding to the news that the investigation had been opened, Ms Bearder told The Independent: “I look forward to seeing the verdict of the Conduct Committee and I expect President Tajani to implement their decision fully.”

“Farage bangs on about people losing trust in politicians, but can the people trust him to declare his financial support correctly?”

Mr Farage told Channel 4 News “no comment” on their investigation, but later told the BBC: “Whatever happened after the referendum – I was leaving politics, it happened mostly in America, it had nothing to do with politics, nothing to do with the Brexit Party, it was purely on a personal basis. I was looking for a new career and a new life – it’s got nothing to do with anything, it’s a purely private matter.”

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However, Mr Farage remained an MEP and continued to draw his MEP’s salary throughout the whole period, meaning the gifts would likely have had to have been declared under transparency rules.

Asked if he had declared the money with the European parliament authorities, he said: “Of course not, it’s a purely private matter, non-political in absolutely every way.”

Mr Banks said: “Channel 4’s attempts to smear myself and Nigel come at a time when the Brexit Party is riding high in the polls, so it should come as no surprise to anyone.”