UKIP's only MP Douglas Carswell has said he is quitting the party.
In a statement on his website, Carswell said he would not defect back to the Tories but would sit in the House of Commons as an independent.
"Like many of you, I switched to UKIP because I desperately wanted us to leave the EU. Now we can be certain that that is going to happen, I have decided that I will be leaving UKIP," he said.
"I will not be switching parties, nor crossing the floor to the Conservatives, so do not need to call a by-election, as I did when switching from the Conservatives to UKIP. I will simply be the Member of Parliament for Clacton, sitting as an independent."
Carswell added: "I will leave UKIP amicably, cheerfully and in the knowledge that we won."
Responding, UKIP leader Paul Nuttall said Carswell was "genuinely committed to Brexit, but was never a comfortable" member of UKIP.
Writing on Twitter, former UKIP leader Nigel Farage wrote that Carswell had "jumped before he was pushed".
"He was never UKIP and sought to undermine us," he said. "He should have gone some time ago."
Carswell has jumped before he was pushed. He was never UKIP and sought to undermine us. He should have gone some time ago.
Farage had previously called on Carswell to quit, accusing him of trying to divide the party.
“As a party, how can we let a man represent us in the House of Commons who actively and transparently seeks to damage us? I think there is little future for UKIP with him staying inside this party," he wrote in the Daily Telegraph in February. “The time for him to go is now.”
In October 2014 Carswell, then the Tory MP for Clacton, triggered a by-election by stepping down and defecting to UKIP. After winning the election he became UKIP's first elected MP, and held on to his seat at the 2015 general election.
Arron Banks, a major backer of UKIP during the Brexit campaign and close ally of Farage, responded to Carswell's announcement by tweeting a smiley face emoji.
😊✅ https://t.co/YgxQj3I2Uo
In his statement, Carswell said that with Britain voting to leave the EU last June, UKIP had "now achieved what we were established to do".
"UKIP might not have managed to win many seats in Parliament, but in a way we are the most successful political party in Britain ever. We have achieved what we were established to do – and in doing so we have changed the course of our country's history for the better. Make no mistake; we would not be leaving the EU if it was not for UKIP – and for those remarkable people who founded, supported and sustained our party over that period," he wrote.
He added: "Cheer up! The days when small elites can try to arrange human social and economic affairs by grand design are coming to an end. Change is coming – Brexit is just the beginning."
Carswell's decision will not only deny current UKIP leader Nuttall – who unsuccessfully stood in the Stoke-on-Trent Central by-election earlier this year – a voice in the Commons, it could also cost the party hundreds of thousands of pounds in short money it was entitled to after winning more than 3.8 million votes at the last general election.
UKIP already faces a funding crisis when Britain leaves the EU in 2019 and its MEPs will find themselves out of a job, while earlier this month Banks quit the party and set up a new organisation called the Patriotic Alliance.
In a statement, Nuttall said Carswell's resignation was "not a surprise".
"I was elected on a pledge to forge unity in the party, and have had many discussions with key players to try and make that happen, but it had become increasingly clear to me that some things were simply beyond reach," he said.
Nuttall added: "Our party has not benefited financially or organisationally from having Douglas in Westminster. With this in mind, his departure will make no difference to my ability or focus on delivering the reforms I promised when elected as leader.
"As we redefine our mission and take up the next phase of our campaign to rebuild a confident, independent nation, Douglas would have been increasingly out of kilter with our members’ aspirations."
The statement concluded: "We now have an opportunity to put behind us the most damaging internal conflict which has dogged us over the past year, and look forward with optimism and unity of purpose to the very real challenges of policing Brexit and further reforming the vigorous democracy of the UK."