Liberal Democrat Sarah Olney has defied the odds and Zac Goldsmith's 23,000 vote majority to beat the independent incumbent and become the new MP for Richmond Park and North Kingston.
The dramatic win comes just a day after former deputy prime minister Nick Clegg told IBTimes UK that his party was set to secure an "astonishing win" over the former Conservative MP.
The by-election was triggered after Goldsmith quit the House of Commons over the government's decision to back a third runway at Heathrow Airport, in west London.
But Brexit was just as big an issue, with the Liberal Democrats taking a pro-EU stance and highlighting Goldsmith's Euroscepticism.
The tactics, which included a last-minute endorsement from musician and campaigner, Sir Bob Geldof, paid off as Olney won 20,510 of the vote against Goldsmith's 18,638 on a turnout of 53.6%.
The result means the Liberal Democrats now have nine MPs in the House of Commons, which is expected to embolden Remain campaigners.
Liberal Democrat MP Alistair Carmichael, speaking after Olney's victory, told IBTimes UK that the voters of Richmond Park had sent a warning to Theresa May about a so called "hard Brexit".
"I get the sense that the 48% who voted to remain in the EU are now feeling completely alienated by May and her government," he said.
"There's a feeling that she does not listen to them and does not respect their views...she has duty to bring the whole back together and she is not doing that. Until she does this sort of thing is going to keep happening."
The Conservatives, who like the Greens and Ukip did not contest the by-election, commiserated Goldsmith on his defeat.
"Zac has been a strong and principled champion for the residents of Richmond Park and North Kingston over the past six years as their Member of Parliament, and a popular figure in the Conservative Party. We are sorry that he is no longer in the House of Commons," a Tory spokesperson said.
"This result doesn't change anything. The Government remains committed to leaving the European Union and triggering Article 50 by the end of March next year.
"In addition, we will continue to take decisive action in the national interest to secure the UK's place the world - supporting a third runway at Heathrow to secure jobs and business opportunities for the next decade and beyond."
Goldsmith, meanwhile, stressed the by-election was not a "political calculation" and wished Olney well in her "very, very important job".
Sarah Olney's full victory speech
"Let me start by thanking the other candidates for a hard-fought campaign – and to Zac Goldsmith in particular, I wish you well and assure you that I will continue your fight against the expansion of Heathrow.
"I also would like to thank the returning officer, the staff that have worked so hard today and yesterday and of course the police. I want to thank my amazing campaign team led by James Lillis and the thousands of volunteers who have taken time to support me over the course of the campaign.
"I want to thank my family and friends for the wonderful support they've given me – particularly my husband Ben and our children. I want to thank our leader Tim Farron, and all the other party members who could not have been more supportive. And I'd like to thank the Greens, More United, the Women's Equality Party and all the other people beyond the Lib Dems who have supported me in this campaign.
"A year and a half ago, I wasn't involved in politics. I wasn't a member of a political party. I'd never been involved in a political campaign. I'd never thought about being a politician. But I knew I was a Liberal – I believed in openness, tolerance, compassion, working with our neighbours at home and around the world – and when I saw what happened at the General Election and I felt I had to get involved.
"I think a lot of people in this community had the same feeling this summer. Richmond Park is full of people like me who felt that something was going wrong. That the politics of anger and division were on the rise. That the liberal, tolerant values we took for granted were under threat. We were seeing the UKIP vision for Britain in the ascendancy – intolerant, backward-looking, divisive; just as we see it in America and across Europe.
"Well today we have said no. We will defend the Britain we love. We will stand up for the open, tolerant, united Britain that we believe in.
"The people of Richmond Park and North Kingston have sent a shockwave through this Conservative Brexit Government. And our message is clear: we do not want a 'hard Brexit'; we do not want to be pulled out of the Single Market; and we will not let intolerance, division and fear win."