The main opposition Labour Party also faced a drubbing in a vote that upended the traditional order of British politics and plunged the country into even more Brexit uncertainty. The big winners were the newly founded Brexit Party led by veteran anti-EU campaigner Nigel Farage and the strongly pro-European Liberal Democrats.
With results announced early Monday for all of England and Wales, the Brexit Party had won 28 of the 73 British EU seats up for grabs and almost a third of the votes. The Liberal Democrats took about 20% of the vote and 15 seats - up from only one at the last EU election in 2014.
Labour came third with 10 seats, followed by the Greens with seven. The ruling Conservatives were in fifth place with just three EU seats and under 10% of the vote.
Scotland and Northern Ireland are due to announce their results later.
Farage's Brexit Party was one of several nationalist and populist parties making gains across the continent in an election that saw erosion of support for the traditionally dominant political parties.
Conservative Foreign Secretary Jeremy Hunt said it was a "painful result" and warned there was an "existential risk to our party unless we now come together and get Brexit done."
The results reflect an electorate deeply divided over Britain's 2016 decision to leave the EU, but united in anger at the two long-dominant parties, the Conservatives and Labour, who have brought the Brexit process to deadlock.
Britain is participating in the EU election because it is still a member of the bloc, but the lawmakers it elects will only sit in the European Parliament until the country leaves the EU, which is currently scheduled for Oct. 31.
Farage's Brexit Party was officially launched in April and has only one policy: for Britain to leave the EU as soon as possible, even without a divorce agreement in place.
Farage said his party's performance was "a massive message" for the Conservatives and Labour, and he said it should be given a role in future negotiations with the EU.
"If we don't leave on Oct. 31, then the scores you have seen for the Brexit Party today will be repeated in a general election - and we are getting ready for it," said Farage.
But the election leaves Britain's EU exit ever more uncertain, with both Brexiteers and pro-EU "remainers" able to claim strong support. Labour and the Conservatives, who in different ways each sought a compromise Brexit, were hammered.
The result raises the likelihood of a chaotic "no deal" exit from the EU - but also of a new referendum that could reverse the decision to leave.
The Conservatives were punished for failing to take the country out of the EU on March 29 as promised, a failure that led Prime Minister Theresa May to announce Friday that she is stepping down from leading the party on June 7. Britain's new prime minister will be whoever wins the Conservative party leadership race to replace her.
The favorites, including ex-Brexit Secretary Dominic Raab and former Foreign Secretary Boris Johnson, have vowed to leave the EU on Oct. 31 even if there is no deal in place.
Most businesses and economists think that would cause economic turmoil and plunge Britain into recession. But many Conservatives think embracing a no-deal Brexit may be the only way to win back voters from Farage's party.
Labour was punished for a fence-sitting Brexit policy that saw the party dither over whether to support a new referendum that could halt Brexit. Labour foreign affairs spokeswoman Emily Thornberry said the party needed to adopt a clearer pro-EU stance.
"There should be a (new Brexit) referendum and we should campaign to remain," she said.
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