Nigel Farage is facing an investigation by European parliament authorities over claims that he failed to declare nearly half a million pounds in gifts from an insurance tycoon under investigation by the National Crime Agency.

It was claimed this 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.

In a letter seen by The Independent, one of the European parliament’s quaestors – MEPs responsible for the body’s financial and administrative matters – calls on the parliament’s presidency to “investigate these apparent contraventions as a matter of urgency”.

The push for an 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.

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“As you will be aware, the code of conduct for members of the European parliament with respect to financial interests and conflicts of interest, in particular Article 6(1) of the implementing measures, makes it clear that members shall disclose their attendance at events organised by third parties where the reimbursement of their travel, accommodation or subsistence expenses, or the direct payments of such expenses, is covered by a third party,” Catherine Bearder, the quaestor who is also a Liberal Democrat MEP, said in the letter to Antonio Tajani, president of the European parliament.

“I can see no reference to any of the reported travel or accommodation subsidies related to Mr Farage’s US tour on any of his declarations of financial interests on the parliament’s website.” She noted that Mr Banks was “currently under investigation by the National Crime Agency over the source of his funding for the Brexit campaign”.

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.

Nigel Farage continues to dodge questions surrounding funding from Arron Banks. A well-placed European parliament official told The Independent an investigation would likely lead to Mr Farage being fined if he was found to have broken the rules. Last year, in a separate run-in with expenses rules, Mr Farage had his MEP salary docked by more than £35,000 to recoup misspent EU funds intended for the staffing of his office.

The letter from the quaestor Catherine Bearder to European Parliament president Antonio Tajani

Ms Bearder told The Independent: “Nigel Farage says he is a man of the people standing up to a corrupt elite, but he has been less than transparent about his lavish lifestyle. I hope there will now be a thorough parliamentary investigation to establish why these alleged trips and subsidies were not declared in accordance with parliamentary rules.”

Nick Aiossa, director of the NGO Transparency International EU, told The Independent: “These revelations of Mr Farage’s US tour to attend the Republican convention and meeting Trump indicate he has not been adhering to the rules on declaring financial interests. The European parliament needs to immediately investigate the matter.”

The Brexit Party has not responded to a request for comment on this story at the time of publication.

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.”

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.”