Britain’s new populist Brexit Party was on course to win the country’s European Parliament elections, according to early results released on Sunday, further roiling the already turbulent politics of a country polarized over its failed effort to leave the European Union.
Led by Nigel Farage, the Brexit Party’s strong performance humiliated the governing Conservatives, whose leader, Prime Minister Theresa May, on Friday announced her resignation after almost three troubled years in power.
The 28 member countries of the European Union voted from Thursday to Sunday for the European Parliament, the bloc’s only directly elected institution, which has extensive powers over legislation. The individual country votes often serve as a gauge on important domestic issues.
The results in Britain were a striking success for a party that has existed for only a few weeks, and for Mr. Farage, the campaigner for British withdrawal from the European Union. He is one of the country’s most divisive politicians but also one of its most effective communicators.
The elections also looked set to deal a crushing blow to the opposition Labour Party as it lost votes to two center-left opponents, the Liberal Democrats and Greens, which both took a clearer position against Brexit and supported a second referendum on the issue.
With many of the votes counted, the Brexit Party was ahead with 31.5 percent of the vote. The Liberal Democrats were second with 20.5 percent, followed by Labour with 14.1 percent and the Greens with 12.1 percent. The Conservatives pushed into fifth position with 9.1 percent of the votes.
The Conservative Party’s dire performance will increase pressure on those campaigning to succeed Mrs. May to take a hard-line approach to Brexit that could result in the country leaving the European Union without any agreement.
By contrast, Labour’s setback is also likely to spur recriminations and prompt calls for it to push more explicitly for another referendum on Brexit.
In effect, voters appear to have flocked to parties that either took an uncompromising stance in favor of Brexit, or those that oppose it altogether — abandoning the Conservative Party and Labour, which have, in their different ways, been trying to offer some sort of compromise.
The results underscore the growing polarization of British politics over Brexit, pointing to yet more political uncertainty and volatility in a country that has been in various degrees of political crisis since 2016, when voters opted in a referendum to leave the European Union.
Mr. Farage’s supporters have been fired up by Mrs. May’s inability to lead Britain out of the bloc, a failure that provided a poisonous backdrop to the European Parliament elections. Mrs. May had promised that the country would leave the bloc before the vote and would not have to participate.
After Mrs. May requested two delays to Brexit, the vote went ahead on Thursday. The results were released late Sunday and early Monday after voting had been completed in all of the bloc’s other 27 nations.
Mrs. May announced her resignation after her Brexit plan was rejected by Parliament three times. Her blueprint aimed to keep Britain inside the bloc’s main economic structures until at least the end of 2020 before breaking away.
It foundered largely over a backup plan tying the whole of the United Kingdom to European customs rules until an alternative system could be found to prevent checks on the Irish border. That idea infuriated Brexit supporters who abhor the idea of staying tied to some of Europe’s rules, possibly indefinitely.
Mr. Farage’s Brexit Party prospered by offering a clear and simple message: that Britain must leave the European Union at the end of October, without any agreement if necessary, whatever the economic costs.
Speaking late Sunday after he was re-elected to the European Parliament, where he has been a member for 20 years, Mr. Farage said that his Brexit Party wanted a role in the discussion on withdrawal from the bloc.
“We want to take responsibility for what’s happening, and we’re ready to do so,” he said, adding that Britain should leave when the next Brexit deadline expires.
“If we don’t leave on October 31, then the scores you’ve seen for the Brexit Party today will be repeated in a general election, and we are getting ready for it,” Mr. Farage said.
Mr. Farage’s hard-line stance is now being echoed by leading contenders to succeed Mrs. May, including Boris Johnson, the former foreign secretary and Brexit campaigner.
However, Philip Hammond, the chancellor of the Exchequer, who is not running for the leadership, on Sunday pointed out that Parliament is clearly opposed to leaving without an agreement.