Former Conservative leadership hopeful 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.
He told the BBC the Tories had moved in a direction "more difficult for me" and he wanted to "get back to real issues".
Mr Stewart was expelled from the Tories in the Commons with 20 other Brexit rebels, but remained a party member.
The MP for Penrith and The Border has now left the party. Announcing his intention to stand for London mayor in a video on Twitter, he said: "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.
Mr Stewart told the BBC he thought the political parties were "becoming more and more extreme and being driven apart" and said there was a "gaping hole in the centre ground of British politics".
"The way to really make change in the modern world is intensely local - through being a mayor, not through being a member of Parliament," he said.
The MP said it was "certainly true" that having the Tory whip withdrawn was a "very important" part in making his decision.
But also that Prime Minister Boris Johnson's tone was "more populist than I am comfortable with", saying: "I have seen the Conservative Party move in a direction which is more and more difficult for me."
He has also written an open letter to Londoners in the Evening Standard, saying he will make a stand against the "mutual insults... lazy habits, half-baked ideas and pointless compromises" of party politics.
Mr Stewart first publicly mentioned his resignation on stage at a charity event at London's Royal Albert Hall on Thursday evening, but it was not widely reported.
Writing for his constituency newspaper, the Cumberland and Westmorland Herald, Mr Stewart said he was "hugely grateful" for the support he had received from members of his local party, but added: "It should be no secret that there are also local party members who would rather I did not run again."
Robert Craig, president of the Penrith and The Border Conservative Association, said: "It's a great shame."
A Conservative Party spokesperson said: "We would like to thank Rory for his hard work and wish him all the best for the future."
A Conservative parliamentary candidate for Penrith and The Border will be selected "in due course", a statement added.
Who is Rory Stewart?
- Raised in Perthshire, educated at Eton and Balliol College, Oxford
- Worked as a summer tutor to Prince William and Prince Harry
- Served in the UK Diplomatic Service and in 2003 became deputy-governor of two Iraqi provinces
- Walked for 21 months across Iran, Afghanistan, Pakistan, India and Nepal, which led to a bestselling book
- Elected as an MP for Penrith and The Border in May 2010
- Former international development secretary and prisons minister
- Remain supporter in the 2016 referendum, he went on to back then prime minister Theresa May's withdrawal deal.
- Dark horse candidate in June's Tory leadership contest with social media "Rory walks" campaign, eventually coming fifth
- Revealed he had smoked opium in Iran, a Class A drug in the UK, later saying it was a "stupid mistake"
- Amid speculation about his future last week, he tweeted: "The only thing I am launching next week is my 4 year old's model boat"
Prime Minister Boris Johnson expelled 21 MPs from the Parliamentary party at the start of September after they rebelled against him in a bid to prevent a no-deal Brexit.
Some long-serving figures - such as Ken Clarke and Sir Nicholas Soames - are planning to stand down at the next election, while others, such as former Chancellor Philip Hammond and former attorney general Dominic Grieve - are reported to be considering standing as independents.
Sam Gyimah, another of the expelled rebels, who has now joined the Liberal Democrats, tweeted that Mr Stewart's decision to step down as an MP showed the Conservative Party's "soul has been captured by those who want to turn it into a nationalist party".
Sad to see my friend @RoryStewartUK is standing down at the next election. No place for moderates in the Tory party. The party will pay lip service to One Nation politics, but it’s soul has been captured by those who want to turn it into a nationalist party. There’s no way back. https://t.co/527KGI5kUs— Sam Gyimah MP (@SamGyimah) October 4, 2019
Former Tory MP Nick Boles, who resigned from the Conservatives earlier this year, tweeted that Mr Stewart's departure showed that the "last rites are being read for moderate One Nation conservatism".
Mr Stewart's former cabinet colleague, Amber Rudd, tweeted, before Mr Stewart announced his London mayoral candidacy, that he was an "outstanding minister" and it was a "loss" to politics.
But Conservative MP Shailesh Vara said Mr Stewart's departure did not make the Conservatives "any less One Nation".
Diehard Remainers leaving the Conservative Party doesn't make the Party any less One Nation. To be clear, there are many of us who believe in Brexit and are proud One Nation Tories.— Shailesh Vara MP (@ShaileshVara) October 4, 2019