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LONDON—Nigel Farage announced his resignation as leader of the UK Independence Party on Monday, saying he had achieved his political ambition after his long and controversial campaign for Britain's exit from the European Union.

The announcement marks the latest upheaval in British politics in the wake of the country's vote to leave the EU, which triggered the resignation of Prime Minister David Cameron and has fueled calls for the leader of the opposition Labour Party to quit.

The referendum result and Mr. Farage's resignation raises an existential question for UKIP, which was formed in the 1990s with the sole purpose of getting the U.K. out of the EU and has grown in large part due to its leader's popularity as an antiestablishment figure. Its increasing influence—which some Conservatives perceived as an electoral threat to their party—helped push the EU referendum on to the political agenda.

Mr. Farage, a member of the European Parliament representing the southeast of England since 1999, said he had never wanted to be a career politician and his aim in politics had always been to get the U.K. out the EU.

"During the referendum campaign I said I want my country back. And what I'm saying today is I want my life back, and it begins right now," he said.

Like some other onetime fringe parties across Europe, UKIP has gained popularity in recent years among people disillusioned with globalization and established political parties, winning 3.9 million votes—12.6% of the total—in the 2015 national elections. UKIP has more members of the European Parliament that any other British party.

Mr. Farage sparked anger during the campaign with suggestions Britain could see a rise in sex crimes by migrants if it stayed in the EU and for a provocative campaign poster showing a line of Middle Eastern-looking migrants above the slogan, "Breaking point: The EU has failed us all." The poster was unveiled hours before a pro-Remain Labour lawmaker was killed in a gun-and-knife attack.

He has denied claims his party is racist, saying an Australian-style points-based migration system based on the skills the U.K. needs would be fairer to minorities than the current system that allows all EU citizens in automatically.

Mr. Farage declined to comment on who might succeed him, but said the future of the party, which he has presented as representing the working class, lies in exploiting turmoil in the Labour Party.

UKIP was sound financially but it needed some reform of its structures and professionalism, he said, without giving details.

Mr. Farage doesn't hold a seat in the U.K.'s Parliament. The one member of his party who does, Douglas Carswell, responded to Mr. Farage's announcement with a smiling-face emoji on his usual Twitter feed.

Mr. Farage briefly quit as UKIP leader in 2015 after failing to win a seat in the national election. But within a couple of days the party said its members had rejected his decision and he would be staying on.

Mr. Farage said he hoped the divisions in the U.K. from the referendum would be healed, saying the country had a brighter future in charge of its own destiny. The U.K. now needed strong leadership and direction as it carves out a new deal with Europe, he said.

"We need a new prime minister who puts down some pretty clear red lines that we are not going to give in on issues like free movement, and we need a prime minister that will not sell us out to what is known as the single market but is effectively a big-business protectionist cartel," he said.

The candidates who have put themselves forward to succeed Mr. Cameron as leader of the Conservative Party and prime minister have said they want to reduce net migration, but haven't made clear by how much or presented a timetable for action.

Among the three top contenders, two have said they wouldn't trigger the formal process for leaving the EU this year and one has said she wants to get the process moving swiftly.

The team that negotiates the U.K.'s exit from the EU should include figures from across the political spectrum who reflect the Brexit vote as well as people from the business community, Mr. Farage said.

"We need to be lobbying the German car industry, the French wine producers, because next year both France and Germany have general elections," he said, adding, "and we need to go global."

The official group advocating Brexit kept Mr. Farage at arm's length during the referendum campaign for fear he would alienate some voters. But in the final weeks of the campaign, Leave campaigners adopted UKIP's focus on immigration after starting with a broader argument including sovereignty and economics.

Write to Nicholas Winning at nick.winning@wsj.com

(END) Dow Jones Newswires

July 04, 2016 20:55 ET (00:55 GMT)

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