A third political party in Britain has been thrown into turmoil over its leadership. Nigel Farage, arguably the architect of Britain's referendum on the EU, resigned on Monday as leader of the U.K. Independence Party.

"My aim in being in politics was to get Britain out of the European Union," Mr. Farage told reporters. "And that is why I now feel that I've done my bit, that I couldn't possibly achieve more than we managed to get in that referendum."

It has been less than two weeks since Britons voted to leave the EU and the country's politics have been shaken to the core. The departure of Mr. Farage from the leadership of the controversial party he helmed for almost a decade came after Prime Minister David Cameron said he was leaving and as Labour leader Jeremy Corbyn fights to keep his job amid a caucus revolt.

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Conservative MPs begin voting Tuesday to select two finalists from among five leadership candidates, with Home Secretary Theresa May, who backed Britain remaining in the EU, up against two Brexit campaigners: Energy Minister Andrea Leadsom and Justice Secretary Michael Gove. Party members will decide among the final two in a postal vote that ends Sept. 9. Meanwhile, Mr. Corbyn has refused to resign and long-time Labour MP Angela Eagle has indicated she will force a leadership vote.

On paper, Mr. Farage's departure may not appear to be very important. UKIP has only one MP in the House of Commons but its impact across the country is substantial. The party received around four million votes in the general election last year and Mr. Farage clearly had a better sense of the growing concern about immigration than politicians from other parties.

On Monday, Mr. Farage said he'd accomplished his political objective. "During the referendum campaign I said I want my country back. What I am saying today is I want my life back. And it begins right now."

Mr. Farage has been the driving force behind UKIP since he helped create the party in 1993 and was elected leader in 2006. It started as a movement to pull Britain out of the EU, but as its popularity grew in recent years Mr. Farage became a strong advocate for controlling immigration and cutting government spending. A former commodity trader, he revelled in taking on the establishment and presenting himself as a hard-drinking, chain-smoking commoner.

"There are two types of people in politics," he told The Globe and Mail in 2012. "There are those in politics to achieve rank and distinction, and there are those in politics who want to change the world. I'm a member of the latter category."

It was the rise of UKIP that forced Mr. Cameron to hold the EU referendum and Mr. Farage became a divisive figure during the campaign, shunned by the official Vote Leave side and drawing criticism for anti-immigrant comments. But in the end he appeared to be more in tune with British voters than Mr. Cameron or Mr. Corbyn.

"You'd have to say he's one of the most influential politicians we've had over the last decade," said Tim Bale, a professor of politics at Queen Mary University of London. "I think maybe he simply kind of reached the limit of where he could get UKIP to and felt, for the moment anyway, he didn't have much more to give."

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The impact of his resignation, and how UKIP adjusts, will be substantial. Votes for UKIP were a deciding factor in many ridings in last year's general election. Labour in particular felt the sting, losing several close races to the Tories because of UKIP.

Now that more than 17 million people voted for Brexit, UKIP had been hoping to seize on that momentum to broaden its base. In an interview just after the referendum, Mr. Farage spoke about the bright future of the party, saying that "a lot of working people want a political party that represents their aspirations and their views and not just the views of a clique who live in north London and who don't know much about the real world."

The challenge will be to find a leader who has the appeal of Mr. Farage, but can attract others who may have been turned off by his rhetoric. "Nigel Farage's resignation represents a key moment in British politics," said Matthew Goodwin, professor of politics at the University of Kent.

"UKIP has only enjoyed success under Farage's leadership so his successor has big boots to fill. Much will depend on the outcome of the Conservative Party leadership election and Britain's negotiations with the EU. If these deliver a prime minister and outcome that are not satisfactory to social conservatives and blue-collar workers in Labour areas then UKIP will have a future, and possibly a rosy one."

And while he has quit the leadership and returned before, Mr. Farage indicated that this time he is leaving for good. He will remain as a member of the European parliament.