The former Tory MP Rory Stewart has announced he will run for the London mayoralty as an independent candidate, after resigning from the Conservative party and standing down from his Penrith and The Border seat at the next election.

The former secretary of state for international development surprised political circles and his Cumbrian constituents with the announcement, made just three months after his bid to become leader of the Conservatives.

Last month he was kicked out of the parliamentary Tory party alongside 20 other rebels for voting against the government and in favour of the Benn act which blocks a no-deal Brexit and obliging the prime minister to request an extension to article 50.

Stewart’s announcement came just six days after he tweeted that his only priority in politics was avoiding a no-deal Brexit, and that “the only thing I am launching next week is my four-year-old’s model boat”.

Stewart, who campaigned for remain in the 2016 referendum, had also previously had to deny that he would defect to the Liberal Democrats amid significant speculation over how he might approach another general election after being barred from running as a Tory.

In an open letter to Londoners, published in the Evening Standard, Stewart wrote that he was running for mayor to try and eradicate “the suffocating embrace of our dying party politics”.

He said political leaders had “retreated to a madhouse of mutual insults in the Gothic shouting chamber of Westminster, pitting one group against another – rich against poor, London against the rest, Brexit against remain – and all the time getting further and further away from compromise, practical solutions, and the centre ground.

“And this is why I’ve decided to stand, not for a party, but as an independent.”

Rory Stewart (@RoryStewartUK) I am running as an Independent candidate for Mayor of London, and here’s why.



Please join me in my campaign: https://t.co/8y3xWpl9hY#Rory4London pic.twitter.com/jnaNy8IF0s

Colleagues said his departure could be a huge loss to politics in Westminster. Amber Rudd, the former works and pensions secretary who has also quit the Tories, described him as “principled”. Former Tory minister Sam Gyimah, who has joined the Lib Dems, said Stewart’s departure showed there was no longer a place for moderate one-nation Conservatives within Tory ranks.

The 46-year-old Eton and Oxford educated Scot served as deputy governor of two provinces in Iraq following the 2003 invasion, and is well known for walking across Afghanistan, documented in his best-selling book The Places in Between.

In government he served as a minister for Defra with responsibility for flood response, a foreign office minister and prisons minister before being promoted to secretary of state for international development.

His approach to the Conservative leadership contest, which involved walking the length and breadth of the country to meet the public, taking awkward selfies and using the hashtag #RoryWalks, electrified the early weeks of the campaign. However, he was knocked out in the third round.

Stewart drew praise for shaking up the party’s image and broadening its appeal to non-Tory voters. Among his media-friendly moves was holding a campaign event in a circus tent.

He chose the Royal Albert Hall in London to effectively announce his departure as a Cumbrian MP at an event where famous letters are read to an audience.

Coming on stage to read a letter from Eton about the then-schoolboy Boris Johnson, sent to his father, Stanley, Stewart told the audience of thousands: “This letter constitutes my resignation from the Conservative party.”

While it was not immediately clear whether his comments should be taken seriously, on Friday morning he tweeted: “It’s been a great privilege to serve Penrith and The Border for the last 10 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.”

He timed his announcement to fit in with the print deadline for his local paper, the Cumberland and Westmorland Herald, based in Penrith, writing how “so much of what I love about Britain lies in Cumbria”.

Just 14 minutes after his 10.30am announcement with his local paper, Stewart revealed his intention to run as mayor of London, leaving local reporters scrambling before their deadline.

Rory Stewart: ‘If Boris Johnson gets a deal, my political career is over’ Read more

Images on his website were swiftly changed from fields of animals to the London skyline.

Some local Conservatives and constituents were said to be disappointed at his move but understood his decision, with many paying tribute to Stewart’s service to the constituency, understanding of the farming community, leading a campaign for rural broadband and his work in the aftermath of Storm Desmond.

Stewart had hinted that some in his local association did not want him to run again in the next election, expected to be held this autumn.

The fformer Lib Dem leader Tim Farron, whose constituency Westmorland and Lonsdale abuts Stewart’s, said: “I’m not surprised really but I am very sad for him … I don’t agree with him on a few things but he is very dedicated to public service and he just feels that the future of the party that he has been in doesn’t provide a home for him.

“I think there was a reasonable chance he could have won Penrith and The Border – he was a very respected local MP. A lot of people not of his political colours voted for him. People were proud of him and that he was their MP.”

Farron said he did not expect Stewart would win the London mayoralty against Labour’s Sadiq Khan, as he may end up splitting the vote from the Tory candidate, Shaun Bailey, and the Lib Dem candidate, Siobhan Benita.

The bookmaker Coral said that after a flurry of bets the odds on Stewart becoming mayor were as short as 2-1.