Nigel Farage, the pro-Brexit campaigner and serial disrupter of British politics, announced plans on Monday to run as a candidate in Britain’s general election next month, dealing a new setback to the prospects of the country’s embattled prime minister, Rishi Sunak.

The surprise announcement from Mr. Farage, who represents an insurgent hard-right movement that campaigns to curb immigration, threatens to upend the campaign by taking votes from Britain’s governing Conservative Party. In doing so, he could make it even harder for Mr. Sunak and his party to narrow a double-digit gap in the polls with the opposition Labour Party.

Divisive, charismatic and famed for his communication skills, Mr. Farage was one of the architects of Brexit, which a slim majority of Britons supported in a 2016 referendum. An earlier decision by Mr. Farage not to run this year was thought by some analysts to have sapped momentum from his party, Reform U.K., the successor to the Brexit Party he once led.

Mr. Farage said last month that he would not seek a parliamentary seat because he wanted to prioritize supporting Donald J. Trump’s electoral campaign in the United States. Mr. Farage is a longtime ally of the former president and campaigned for him in 2016 and 2020.

But on Monday Mr. Farage reversed his decision, saying he would take over as leader of Reform U.K. for the next five years and run for a seat in Parliament.

“I’ve changed my mind — it’s allowed, you know,” he said. “I am going to stand in this election.” He added that he would run in Clacton, a seaside area where support for Brexit has been strong.