Mr. Farage has said that he did not break any rules and that he was under no obligation to declare the money because it was a personal gift he received before his successful 2024 bid for Parliament.

His political opponents argue that the gift, which came from Christopher Harborne, a supporter of Reform U.K. who lives in Thailand, should have been declared after his election. The case was referred by the main opposition Conservative Party to the Parliamentary Commissioner for Standards, an official watchdog. The gift was first reported by The Guardian, which said Mr. Farage received it and “within weeks” announced he would stand in the election.

The office of the commissioner did not respond to a request for comment.

The code of conduct for the House of Commons states that new lawmakers “must register all their current financial interests, and any registrable benefits (other than earnings) received in the 12 months before their election.”

While “purely personal gifts,” are exempt, the code adds, “both the possible motive of the giver and the use to which the gift is to be put should be considered. If there is any doubt, the benefit should be registered.”

Lawmakers found to have broken the code of conduct can be asked to apologize or, in more serious cases, face punishments including suspension from Parliament. Sometimes this can lead to them having to run in an election to keep their seat.

Reform U.K., which has led opinion polls in Britain for more than a year, took seats from the Conservative Party and from Labour in last week’s elections. Its success in the past two years has upturned British politics, threatening both Labour and the Conservatives, the traditionally dominant parties.

The party won more than 1,400 seats on municipal councils across England in elections last week, a resounding triumph over the governing Labour Party that has prompted calls for the resignation of Britain’s prime minister, Keir Starmer, and speculation about Reform’s bright future.

However, its lead in the polls, which rose to about 30 percent of voters last year, has fallen to around 26 percent in recent months, and the party’s vote share in the most recent elections was similarly down compared to local elections last year, according to an analysis by the polling expert John Curtice. That has led some political analysts to suggest the party’s support might have peaked.

Mr. Farage said the gift is being used to pay for a lifetime of personal security. “I’ve been the most attacked, physically, politician of modern times,” Mr. Farage told broadcasters last month.

“And yet despite repeated requests to the Home Office, the police, for protection and help, I’ve been denied at every twist and turn,” he added. “This money is the only way I can look after myself, and protect myself for the rest of my life.”

Last year, Mr. Harborne, a British cryptocurrency investor, gave a donation of £9 million to Reform U.K., the largest single donation to a British political party by a living person. He had previously donated to the Conservative Party as well as the Brexit Party, the forerunner of Reform U.K.

Critics of Mr. Farage have pointed out that since he entered Parliament, he has championed the cryptocurrency industry and pushed for light-touch regulation. In November he wrote an opinion essay for a British newspaper with the headline, “Reform’s Britain will lead the digital money revolution.” He wrote, “Crypto is no longer a fringe experiment.”

He has also pledged to introduce a bill to make Britain “the world’s premier hub for cryptocurrency and blockchain innovation.”

Reform U.K. did not respond to a request for comment. In a statement given to the BBC, the party said, “Mr. Farage’s office is in communications with the Parliamentary Commissioner for Standards,” adding, “He has always been clear that this was a personal, unconditional gift and no rules were broken. We look forward to this being put to bed once and for all.”

In January, Mr. Farage was found by the standards commission to have failed to register £384,000 in income from GB News, Google, X and the Cameo app on time, but he was allowed to update his declaration without any further action being taken. The standards commissioner, Daniel Greenberg, concluded that it had been an “inadvertent” breach.

In a statement, the Conservative Party said that £5 million was “more than most people will earn in a lifetime,” adding, “Nigel Farage needs to explain how he got it, why he got it, and why he didn’t declare it.”

It said, “If there is a simple answer then he should welcome these investigations. But like so often with Reform, there is something very fishy about the whole story.”

Anna Turley, chair of the Labour Party, said in a statement that “Nigel Farage has been avoiding legitimate questions since news of his billionaire backer’s ‘gift.’ It’s right that he faces a proper investigation.”