UKIP’s Douglas Carswell is quitting the party to become an independent MP and says he is doing so “amicably”.
The party’s only MP defected from the Conservatives to UKIP in 2014.
Former leader Nigel Farage recently called on Mr Carswell to quit accusing him of “actively working against UKIP”.
A spokesman for UKIP donor Arron Banks, who had suggested he would stand against Mr Carswell, said a by-election should be called and the Clacton MP had jumped before he was pushed.
UKIP Leader Paul Nuttall said Mr Carswell’s announcement was “not a surprise”.
He said: “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.
“Douglas was genuinely committed to Brexit, but was never a comfortable Ukipper.”
Mr Banks’s spokesman Andy Wigmore said: “The net has been closing in – there is a UKIP National Executive Committee meeting on Monday and he knew he was for the chop, so jumped.
“He should call a by-election and Arron will stand against him.”
Mr Banks previously indicated he could stand against Mr Carswell at the next election amid a long-running feud over UKIP’s future direction and strategy.
On Twitter, Mr Banks posted a smiley face emoji and a green tick.
Mr 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.”
Mr Carswell chose to announce the decision on his blog, saying: “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 added: “I will not be switching parties, nor crossing the floor to the Consevatives, 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.
“I will leave UKIP amicably, cheerfully and in the knowledge that we won.”
Speaking to the BBC News Channel, Mr Carswell said he would will still be sitting in opposition “holding the government to account”.
He declined to comment on where his decision left UKIP, saying there was a “political cartel” in the UK and he wanted to be part of “far-reaching change” as an independent MP.
Mr Carswell said: “I think the fundamental reason for UKIP – certainly the reason that I made that switch and I think the reason why millions of people around the country made the switch to UKIP, was over the Europe question.
“And we can be sure now, we can be absolutely certain, Brexit is in good hands, we are going to leave and all of the things that Vote Leave campaigned for are going to come to pass.”
Mr Carswell, 45, first entered the Commons in 2005 as MP for Harwich, defeating Labour’s candidate by just 920 votes. By 2010 he defeated the same opponent by 12,000 votes – although boundary changes had seen the seat renamed Clacton.
After defecting to UKIP he stood down to seek re-election in a by-election, which he won by 12,404 votes in October 2014. At the general election in 2015, he retained the seat with a 3,437 majority.
UKIP MEP Bill Etheridge said he was “delighted” at the announcement.
“I believe was not genuinely interested in representing UKIP in Westminster but has been the focus of serious infighting in the party has gone.
“But I think he needs to step down and call a by election… He was elected twice on a UKIP ticket, with UKIP resources and the hard work and shoe leather of UKIP activists.”
Earlier this month, Mr Carswell had vowed to “absolutely” fight the next general election as a UKIP candidate.
He was speaking after UKIP leader Paul Nuttall urged senior figures to “stick together”. Mr Nuttall, who unsuccessfully stood for Parliament at the Stoke Central by-election, said he needed time to sort out the “mess” his party was in.