Former Conservative leadership candidate Rory Stewart is quitting as an MP to run for London mayor as an independent candidate.
He will stand in next year’s election against current Labour mayor Sadiq Khan and Tory candidate Shaun Bailey.
Mr Stewart tweeted it had been a “great privilege” to serve Penrith and The Border for the last 10 years.
He was expelled from the Tories in the Commons with 20 other Brexit rebels, but remained a member of the party.
Mr Stewart announced his intention to stand for London mayor in a video on Twitter, saying: “I’m leaving that gothic shouting chamber of Westminster.
“I’m getting away from a politics which makes me sometimes feel as though Trump has never left London and I want to walk through every borough of this great city to get back to us on the ground.”
His Conservative rival, Mr Bailey, said he welcomed “any candidate’s decision to stand and hold Mr Khan to account over his woeful record in London”.
He said he would “continue to focus on serious violent crime and how we’re going to get a grip on the violence on our streets”.