“I have decided that the people of Clacton should be the judges of my actions,” Mr. Farage said in a statement broadcast on his party’s YouTube channel. “This will be a ‘people versus the establishment’ by-election,” he added, referring to a special parliamentary election.

The success of Mr. Farage, an architect of Brexit whose anti-immigration party has led in opinion polls for over a year, was instrumental in destabilizing Prime Minister Keir Starmer, who announced his resignation last month.

But Mr. Farage’s growing profile and the possibility that he could become prime minister at the next general election has attracted scrutiny of his financial affairs.

In May, The Guardian revealed that he had received an undisclosed gift of 5 million pounds (about $6.7 million) from a cryptocurrency billionaire, Christopher Harborne, a Briton who lives in Thailand.

Mr. Farage argues that the gift was unconditional, that it was made before he won a seat in Parliament and that there was no requirement to declare it. However, Daniel Greenberg, Parliament’s standards commissioner, has opened an investigation into whether the money should have been made public under rules that require new lawmakers to declare some financial benefits received in the 12 months before their election.

Over the weekend, The Sunday Times of London reported that Mr. Farage had separately failed to declare benefits provided by a political ally, George Cottrell, who served eight months in prison in the United States on the fraud conviction.

According to the newspaper, Mr. Cottrell’s support included providing social media staff who worked for Mr. Farage in the year before he was elected, as well as the use of a property rented by Mr. Cottrell near Buckingham Palace.

Mr. Farage insisted he followed all of the rules and has accused journalists of hounding his family and “despicable behavior.”

He also suggested on Tuesday that the Sunday Times article had now triggered a second investigation, saying, “Despite the fact that many of the things that were written in the article were inaccurate or irrelevant, yet another standards investigation is underway.”

If Mr. Greenberg finds that the gift from Mr. Harborne should have been declared, Mr. Farage might, under parliamentary rules, have been suspended from Parliament and forced to fight for re-election in Clacton.

But his announcement on Tuesday effectively pre-empts that scenario. It does not, however, block the investigation, which could be suspended pending the special election in Clacton and then resumed if Mr. Farage wins. That raises the possibility that the Reform U.K. leader could be forced into a second contest in Clacton later in the year.

In comments to reporters on Tuesday, Mr. Starmer described Mr. Farage’s move as “a desperate stunt,” from a politician “up to his neck in sleaze.” Mr. Starmer’s likely successor, Andy Burnham, called it a “gimmick designed to distract from serious allegations about Farage’s funders.”

Despite Reform’s success in May in local elections, the party has also suffered some reverses. Last month, it lost a crucial special parliamentary election in Makerfield, in northwest England, which was won decisively by Mr. Burnham.

This year it also lost a special election in nearby Gorton and Denton to an insurgent Green Party. And last year, it suffered a similar fate in Caerphilly in Wales, in a special election to the Welsh Parliament, which was won by the center-left nationalist Plaid Cymru party.

In national polls, Reform’s support has fallen from about 30 percent last year to about 25 percent now, with Labour and the Conservatives at around 20 percent each.

Reform has recently faced competition on its right flank from a far-right party called Restore Britain, which was founded by a former ally of Mr. Farage.

Mr. Farage, a highly effective campaigner and a longtime disrupter of British politics, will be confident of winning the Clacton seat again, having achieved a majority of 8,405 votes in the 2024 general election. Polling suggests he remains popular in the area, which voted for Brexit by a large majority in 2016.

But on Tuesday, Kemi Badenoch, the Conservative Party leader, said she would not run a candidate in what she called a “fake” special election. Her party would, however, contest any vote that followed the investigation by Mr. Greenberg, she said. The centrist Liberal Democrats and Restore Britain gave similar indications.

And Mr. Farage’s gambit seems unlikely to end the scrutiny of his finances ahead of the next general election, which must take place by 2029.

His comments, made in a video interview with the British outlet The Sun, came as the British Parliament’s standards watchdog confirmed that Mr. Farage was under investigation for not declaring receipt of the £5 million (about $6.7 million), which came from Christopher Harborne, a Briton who lives in Thailand.

Mr. Farage argues that the gift was unconditional, was made before he won a seat in the general election in 2024, and that there was no requirement to declare it.

Critics say that under parliamentary rules the money should have been registered after the election, and the main opposition Conservative Party referred the case to the Parliamentary Commissioner for Standards.

Reform U.K. has led in the opinion polls since early last year and won more than 1,400 seats on municipal councils across England in elections last week. That performance has prompted calls for the resignation of Britain’s prime minister, Keir Starmer, and speculation that Mr. Farage might himself enter Downing Street after the next general election, which must take place by 2029 but could come earlier.

But Reform U.K.’s success has brought closer scrutiny and criticism that Mr. Farage, who cultivates the image of a political outsider, depends on wealthy donors.

Last year, Mr. Harborne, the cryptocurrency billionaire who gave Mr. Farage the gift, also gave a donation of £9 million to Reform U.K., the largest single donation to a British political party by a living person. Critics of Mr. Farage have pointed out that since he entered Parliament, he has championed the cryptocurrency industry and pushed for light-touch regulation.

Asked last month about his gift to Mr. Farage, Mr. Harborne told The Telegraph that he “wasn’t expecting anything in return apart from ensuring his safety,” adding, “I gave him the money because of my great admiration for the decades of work he had done to achieve Brexit.”

In The Sun interview, Mr. Farage said he was “not in the least bit concerned” about the parliamentary inquiry.

“This was given to me on an unconditional basis — completely unconditional basis — but frankly, it was given as a reward for campaigning for Brexit for 27 years,” he said. Mr. Farage led two parties before Reform U.K., both of which opposed Britain’s membership in the European Union. He was also a prominent campaigner in the 2016 referendum in which a narrow majority of Britons voted to leave the bloc.

Last month, Mr. Farage told broadcasters that the gift was to pay for a lifetime of personal security. “I’ve been the most attacked, physically, politician of modern times,” he said.

Reform U.K. said in a statement that Mr. Farage’s new comments did not contradict his earlier ones because “both can be true at the same time.” The gift was “a reward after the years of danger he’s put himself in and is still in now,” it added.

Responding to a separate report by Sky News that Mr. Farage bought a £1.4 million property in cash shortly after receiving the £5 million gift, the party said that “the offer and purchase process for the property commenced before the gift,” adding that as part of that process, he had already demonstrated he had the funds.

Anna Turley, the chairwoman of the governing Labour Party, said in a statement that Mr. Farage had “repeatedly dodged questions on his multimillion-pound ‘gift,’” adding, “Now we can see why — this totally stinks.” She called on Mr. Farage to “come clean with the public as to what this £5 million was used for and why he failed to declare it.”

Ahead of the 2024 general election, Mr. Farage initially said he did not intend to run for a seat but changed his mind about a month before the vote. The Guardian, which first reported the £5 million gift, said the money was received “shortly before” Mr. Farage announced he would run.

Asked in his interview with The Sun whether the gift played any role in his change of heart, Mr. Farage rejected the idea.

“I can’t be bought by anybody,” he said citing his public falling out with Elon Musk, the technology billionaire. “He wanted to give us a load of money if I said certain things publicly and I refused,” Mr. Farage said referring to Mr. Musk without elaborating.