Douglas Carswell has announced he will quit Ukip, leaving the party with no MPs, with former leader Nigel Farage saying he "jumped before he was pushed."
Carswell will continue to sit as an MP, a statement on his website said. He will not call a by-election as he will not be changing political party, he said.
Ukip has been riven since the referendum by infighting, with Farage calling for Carswell to be thrown out of the party.
Read more: Carswell explains why he's quitting Ukip
Farage on Twitter said "He was never UKIP and sought to undermine us. 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.— Nigel Farage (@Nigel_Farage) March 25, 2017
The MP for Clacton did not address the rivalry in the party, and did not mention any Ukip politicians by name, instead saying he had decided to leave because the party had served its function.
He said: "I will leave Ukip amicably, cheerfully and in the knowledge that we won."
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.
Carswell thanked the "heroic efforts of Ukip party members and supporters," saying they "helped Vote Leave win the referendum" in a pointed reference to the officially designated campaign group, which was not backed by his party.
Ukip's leadership, including Nigel Farage and billionaire backer Arron Banks, focused their campaigning during the referendum on alternative group Leave.EU.
Carswell's brand of libertarian Brexit stood increasingly at odds with the anti-immigration message espoused by Leave.EU and Ukip.
Ukip's current leader, Paul Nuttall, said: "Douglas was genuinely committed to Brexit, but was never a comfortable Ukipper."
He added: "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."
Farage's spat with Ukip's sole MP spilled out into the public after reports suggested Carswell moved to block a knighthood for the former leader and sitting MEP. However, Farage said in an article for a newspaper the knighthood was not the reason for his anger, suggesting Carswell had damaged Ukip.
After the article at the start of March Carswell told the Guardian he was "Ukip 100 per cent."
Carswell's defection comes after Banks announced he will cut ties with Ukip to try to set up his own political movement. Banks, an insurance billionaire, has recently set up a website, modelling itself after the far-right Breitbart news site.
Banks greeted Carswell's exit from the party on Twitter.
Carswell joined the party in 2014, heaping pressure on the Conservative party at a time when splits over Europe were beginning to be exposed.
Carswell managed to win the Clacton seat for Ukip in a 2014 by-election from his former Conservative party. He first joined Parliament in 2005 as a Conservative before the constituency boundaries were readjusted.