Douglas Carswell has quit Ukip to leave the Eurosceptic party without a single MP.
The representative for Clacton in Essex said he would remain his seat as an independent.
Mr Carswell said: “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 hailed the "extraordinary achievement" of pulling Britain out of the EU, and said: "By April 2019, Britain will no longer be a member of the EU. After twenty-four years, we have done it. Brexit is in good hands."
Mr Carswell added: "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."
It comes after a public spat in which Ukip’s former leader Nigel Farage accused Mr Carswell of "working with his Tory friends" to undermine the party.
Mr Carswell was first elected in 2005, and defected from the Conservatives to Ukip in 2014, triggering a by-election.
His announcement of his decision to leave Ukip pointedly did not include any reference to Mr Farage, with whom he has endured a rocky relationship, or current leader Paul Nuttall.
Instead, hailing the role of grassroots volunteers, he 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."