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.
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.
Writing on his blog, Mr Carswell said: “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.
“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.
“I will leave UKIP amicably, cheerfully and in the knowledge that we won.”
Mr Carswell said: “I will be putting all of my effort into tackling some of the local problems affecting the NHS in our part of Essex… Local comes first.”
Speaking to the BBC, Mr Carswell 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, 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.
“It’s a lovely, sunny day and someone 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.
Douglas Carswell quitting UKIP to become independent MP for Clacton}