Why has the Tory majority of 16,000 votes been overturned?
The leader of the Liberal Democrats has said the party's historic win in the Chesham and Amersham by-election will "send a shockwave through British politics" as the prime minister called the loss "disappointing".
Sir Ed Davey celebrated the result by destroying a "blue wall" representing Tory southern seats after the Conservative Party suffered a humiliating defeat in the contest.
Sarah Green is the country’s newest MP after winning the seat, which has been a Conservative stronghold since its creation in 1974.
Boris Johnson said it was "certainly a disappointing result."
"There were particular circumstances there and we are getting on with delivering our agenda for the whole country, that’s what One Nation conservatism is all about."
Liberal Democrat leader Ed Davey celebrates the victory.
Chesham & Amersham result:
Lib Dems: 56.7% (+30.4)Conservatives: 35.5% (-19.9)Green Party: 3.9% (-1.6)Labour: 1.6% (-11.2)
The contest was triggered by the death of former Cabinet minister Dame Cheryl Gillan, who took the seat with a majority of 16,233 in the 2019 general election – some 55% of the vote.
The stunning result saw Ms Green defeat Tory Peter Fleet by a majority of 8,028.
Ms Green said she was “humbled by the faith you have placed in me” and promised she would hold the Government to account.
Liberal Democrat Sarah Green's winning speech:
“This Conservative Party has taken people across the country for granted for far too long,” she said.
"The voice of Chesham and Amersham is unmistakable. Together we have said 'enough is enough, we will be heard and this government will listen.'
"This campaign has shown that no matter where you live, or how supposedly safe a constituency may appear to be, if you want a Liberal Democrat member of parliament, you can have a Liberal Democrat member of parliament.
The newly-elected MP also took to Twitter to give her thanks, claiming her party "sent a message to the whole country tonight."
Defeated Tory candidate Peter Fleet said: “It’s clear that on this occasion the constituents here in Chesham and Amersham have chosen a different candidate to represent them, to serve as their Member of Parliament."
He continued: “I look forward to how we can start to rebuild that trust and understanding amongst all those people in Chesham and Amersham and as far as the Conservative Party is concerned that work actually starts in the morning.”
Mr Davey said the result was "sending a message to the whole of the country".
"Liberal Democrats came here as the underdog but we campaigned hard and we are sending a shockwave through British politics.
"This was one of our best ever by-election results…this is a great result for the Liberal Democrats.
"A huge swing to us and, do you know what, I think there are many conservative MPs across the country who are now worried."
Boris Johnson reacts to the result:
The result was also one of the worst in Labour's history, with the party gaining so few votes they lost their deposit.
Labour shadow minister Jess Phillips said the voters were "clever" and had voted tactically to beat the Conservatives in the election.
She told ITV News: "Voters are clever and they decided that they wanted to send a message to the government about how they weren’t happy with a variety of different things.
"I think they corralled around the candidate they thought had the best chance to beat the conservatives.
"Tactical voting and a squeeze message by the Lib Dems is absolutely bread and butter," she added.
The result was so unexpected that Financial Times Chief Political Correspondent, Jim Pickard, said he would eat his hat if the Conservatives lost the election.
On Friday, he posted a video of him with the single word "penance", which shows him cutting up and eating a small portion of his hat.