Rory Stewart is going to stand as an independent candidate to be Mayor of London, he has announced.
Earlier today the Conservative MP confirmed he will resign as an MP at the next general election.
Speaking to ITV News Political Correspondent Paul Brand, Mr Stewart said the atmosphere of Parliament was one of the reasons behind his decision.
He said: "One of the things that really convinced me I was doing the right thing was last Wednesday in the House of Commons and watching the Prime Minister's response to the Supreme Court judgement.
"And just the whole atmosphere of Parliament I think it's shameful."
He said he wants to get back to talking to people on the streets and having "direct control" over trying to improve and fix London.
Describing sessions in Parliament as "deeply depressing", he said the House of Commons has become a "shouting chamber".
On Brexit, Mr Stewart, who voted to remain but supported Theresa May's Withdrawal Agreement, said he wants to protect London from any kind of Brexit.
"I'm not running to be a national politician, I'm not in charge of delivering Brexit," he told ITV News.
"I'm in charge of defending London against any type of Brexit.
"Anybody who pretends to you they know whether there is going to be a no-deal Brexit or a soft Brexit or no Brexit at all is deluding themselves, we have no idea."
He also admitted his entry to the mayoral race might affect Conservative candidate Shaun Bailey's chances but said he is "unfortunately a very, very long way behind" Sadiq Khan in the polls.
The best hope of anyone beating Sadiq Khan is him, Mr Stewart claimed.
In a video posted on his Twitter account, Mr Stewart said: "I'm leaving that Gothic shouting chamber of Westminster, I'm getting away from politics which makes me sometimes feel as though Trump has never left London.
"I want to walk through every borough of this great city to get back to us on the ground, making change local and showing that the way we do it is not through division but through love."
Announcing his resignation as an MP on Twitter, Mr Stewart said: "It’s been a great privilege to serve Penrith and The Border for the last ten years, so it is with sadness that I am announcing that I will be standing down at the next election, and that I have also resigned from the Conservative Party."
Mr Stewart served in Theresa May's cabinet and was one of 21 Tory rebels who had the whip removed by Mr Johnson after defying party line in the Commons by backing a move designed to block a no-deal Brexit.
Mr Stewart campaigned strongly against a no-deal Brexit, putting him at odds with the current leadership who want to leave with or without a deal by October 31.
He served as international development secretary until his resignation from government in July, shortly before Mr Johnson became the prime minister.
Former cabinet colleague Amber Rudd tweeted: "What a loss to politics. An outstanding MP & Minister. One of the strongest speakers in Parliament. Principled, patient, thoughtful. I feel certain he'll be back."
Ruth Davidson, who announced her resignation as the Scottish Conservative leader earlier this year, said: "Very sorry to see. Our party should have room for talented, experienced and committed people from across the Conservative spectrum."
Former Tory MP Nick Boles, who resigned from the Conservatives earlier this year, tweeted: "Last rites are being read for moderate One Nation conservatism. @RoryStewartUK joins @RuthDavidsonMSP @SamGyimah @AmberRuddHR and many others. A sad day for British politics but a personal liberation for Rory who will go on to greater things, no doubt."
Former Liberal Democrats leader Tim Farron, the MP for neighbouring Westmorland and Lonsdale, tweeted: "This is a huge loss. Rory has been a strong voice for Cumbria and for decency, moderation and common sense."