Ukip's only member of parliament has quit the party to become an independent MP.
Douglas Carswell, the member of Clacton, has been at the heart of disputes with former party leader Nigel Farage for months, but said he was leaving Ukip "amicably".
Carswell, formerly a Conservative before joining Ukip, said he was leaving the party as it achieved its founding aim of taking Britain out of the European Union.
In a statement on his website announcing the departure on Saturday (25 March), Carswell said: "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 added: "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."
Carswell joined Ukip in 2014 stating that the Conservative party was not "serious about the change that Britain so desperately needs", and frequently campaigned alongside Farage in favour of the Leave vote.
However, since Brexit has been confirmed, Farage has been among Carswell's most vociferous critics, calling for him to be thrown out of the party.
Their falling out is believed to be over Carswell's lack of support for Farage's knighthood bid, in which Carswell went on to mock Farage's failed attempt.
Following Carswell's announcement to leave the party, Farage tweeted: "Carswell has jumped before he was pushed. He was never UKIP and sought to undermine us. He should have gone some time ago."
Ukip leader Paul Nuttall told the BBC Carswell's announcement 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.
"Douglas was genuinely committed to Brexit, but was never a comfortable Ukipper."
In his statement, Carswell confirmed he would not be rejoining the Conservatives, among rumours he has maintained a close relationship with his former party.
Carswell 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."
When Carswell was asked in early March if he was leaving Ukip, he said: "I am 100% Ukip and will be staying with Ukip."