Former UKIP - and now independent - MP Douglas Carswell says he will not be standing in the UK general election.
The Clacton MP said he would be backing the Conservative candidate in the constituency at 8 June's poll.
He was the only UKIP candidate to be elected as an MP at the 2015 general election.
But he quit the party last month after falling out with its leadership. UKIP donor Arron Banks had vowed to stand against him in Clacton.
Mr Carswell defected from the Conservatives to UKIP three years ago but clashed with then leader Nigel Farage and other senior figures over the direction the party was taking.
UKIP leader Paul Nuttall said at the time that Mr Carswell was "committed to Brexit, but was never a comfortable Ukipper".
In a statement on his website, Mr Carswell said: "I intend to vote Conservative on June 8th and will be offering my full support to whoever the Clacton Constituency Conservatives select as their candidate.
"It is sometimes said that all political careers end in failure. It doesn't feel like that to me today. I have stood for Parliament five times, won four times, and helped win the referendum last June. Job done. I'm delighted.
"Having been an MP for about half my adult life, I believe that it is time for me to move on to other things."
Suzanne Evans, seen as one of Mr Carswell's former allies in UKIP, said she was "very surprised" by his decision not to seek re-election.
She told the BBC's Daily Politics it might be "a little bit premature", as the UKIP candidate for Clacton has not yet been selected.
Arron Banks - who described Mr Carswell's departure from the party as "a rare piece of good news for UKIP" - was planning to stand against him in Clacton, although it is not clear whether he would be UKIP's candidate.
When he quit UKIP last month, 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."