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”.
Labour leader Jeremy Corbyn tweeted: “Rory Stewart wholeheartedly backed Tory cuts that have ripped the heart out of our communities and done so much damage to our police, NHS and schools. He would be a disaster for London.”
It comes after Mr Stewart’s announcement that he was stepping down as an MP at the next election and quitting the Conservative Party.
The next scheduled general election is in 2022, but it is widely anticipated a snap poll is imminent, with the prime minister urging MPs to support his call for one. The London mayoral election will be held on 7 May, 2020.
In an interview with London’s Evening Standard, Mr Stewart said: “It’s always difficult to run against your own party. It’s been a painful journey for me. I suppose it was really crystallised when I had the Conservative whip removed.
“I’ve been proud to be a member of the Conservative Party. There are many values I share with it. I parted company largely over Brexit and the tone of the party, which has become increasingly aggressive.”
He has also written an open letter to Londoners in the paper, saying he will make a stand against the “mutual insults… lazy habits, half-baked ideas and pointless compromises” of party politics.
Mr Stewart first publically mentioned his resignation on stage at a charity event at London’s Royal Albert Hall on Thursday evening, but it was not widely reported.