LONDON — The short and happy life of the United Kingdom Independence Party, or UKIP, may be coming to a close after its only member of Parliament, Douglas Carswell, quit the party on Saturday to become an independent.
Mr. Carswell, 45, was considered a great catch, defecting to UKIP from the Conservative Party in August 2014 because, he said, he believed that the Conservative government was ambivalent about holding a referendum to leave the European Union.
In a message on his website on Saturday, however, Mr. Carswell told his constituents: “Like many of you, I switched to UKIP because I desperately wanted us to leave the E.U. Now we can be certain that that is going to happen, I have decided that I will be leaving UKIP.”
The Conservative government, under Prime Minister Theresa May, is set to invoke Article 50 to carry out Britain’s departure from the European Union, known as Brexit, undermining UKIP’s raison d’être.
UKIP received 12.6 percent of the national vote in the 2015 parliamentary election, but it was squeezed, as all third parties are, by Britain’s first-past-the-post system. The party retained only Mr. Carswell’s seat.
But the threat of UKIP to Conservative seats helped push the prime minister at the time, David Cameron, into calling the referendum last June. Mr. Cameron wanted Britain to remain in the bloc, but Britons voted to leave, 52 percent to 48 percent, pushing him to quit politics.
It was a great victory for UKIP, its leader at the time, Nigel Farage, and its financier, Arron Banks. With many Labour Party voters ignoring the unenthusiastic advice of the party’s far-left leader, Jeremy Corbyn, to remain, it seemed that UKIP had a clear path to a future beyond the referendum among working-class voters, especially in England.
But the party has always been a shambles. Mr. Farage quit as leader; his eventual replacement, Paul Nuttall, recently lost a by-election he probably should have won; and Mr. Banks spoke of founding a replacement party or movement.
Saturday was the 60th anniversary of the Rome Treaty that led to the European Union, and a decent number of demonstrators gathered in London to support the bloc and protest Britain’s departure from it. On Wednesday, Mrs. May plans to formally notify the bloc that Britain will quit, beginning divorce negotiations that may run for two years.
Mr. Carswell, representing the coastal constituency of Clacton, was openly critical of Mr. Farage and Mr. Banks, with Mr. Farage urging him to quit UKIP so the party could concentrate on an anti-immigration policy.
On Saturday, Mr. Farage said on Twitter: “Carswell has jumped before he was pushed. He was never UKIP and sought to undermine us. He should have gone some time ago.”
In his note, Mr. Carswell said that UKIP had achieved its founding aims with the vote to leave the European Union: “After 24 years, we have done it. Brexit is in good hands,” he said.
Mr. Banks called on Mr. Carswell to face him in an election. But that is unlikely, and Mr. Carswell, having quit the Conservatives to join UKIP and then quit the UKIP, will carry on alone.