The Conservatives have suffered a humiliating by-election defeat as the Liberal Democrats secured a historic win in Chesham and Amersham.
Sarah Green is the country's newest MP after winning the seat, which has been a Conservative stronghold since its creation in 1974.
The contest was triggered by the death of former Tory cabinet minister Dame Cheryl Gillan, who took the seat with a majority of 16,233 at the 2019 general election - some 55% of the vote.
The stunning result on Thursday night saw the Lib Dems' Ms Green defeat Conservative candidate Peter Fleet by a majority of 8,028.
Liberal Democrat leader Sir Ed Davey said the Tories "blue wall" was "beginning to crumble".
"This will send shockwaves through the British political system," he told Sky News.
"This Liberal Democrat victory was one of our best ever by-election victories. And, if it was repeated across the south of England, literally dozens of Conservative MPs would lose their seats to the Liberal Democrats."
Sir Ed claimed voters in Chesham and Amersham felt like they had been "taken for granted" by the Tories and that many "aren't very happy with Boris Johnson".
"In areas like this it's really clear that it's the Liberal Democrats who can beat the Conservatives," he added. "And boy did we do that last night!"
The 25-point swing from the Conservatives to the Lib Dems was the third-highest since two by-elections in 1993.
Ms Green said she was "humbled by the faith you have placed in me" and promised she would hold the government to account.
In her acceptance speech, 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.
"If you wish to reject Conservative mismanagement and vote for a voice that will represent you and stand up for your rights then it is the Liberal Democrats who will continue to fight your corner."
Government minister Kit Malthouse admitted the by-election result was "very disappointing" for the Conservatives and said there would have to be a "fairly significant post-match analysis" on what went wrong for the Tories.
"It's obviously very disappointing, can't pretend anything otherwise," he told Sky News.
"Really sad for our fantastic candidate Peter Fleet. I was there myself just a couple of days ago, knocking on doors for him.
"We'll obviously have to have a fairly significant post-match analysis and understand what has actually gone on.
"Traditionally, it's always tough for governments, whatever 11 years in, mid-term, to win a by-election, but there are lessons there for everybody and no doubt they will become clear in the days to come."
Mr Fleet acknowledged the need to rebuild "trust and understanding" with voters and also suggested the Tories had been out-campaigned by the Lib Dems.
"Clearly this was a very disappointing result, not the result that I was expecting nor my team," he told reporters.
"It's an absolutely extraordinary result which must take into account the fact that the Liberal Democrat party didn't just throw the kitchen sink at this constituency, I think it was the microwave, the table, the oven, the dishwasher, the dog, the cat and anything else that was lying around as well.
"And we should consider that when we reflect upon the extraordinary nature of the result."
The Chesham and Amersham result has also brought fresh scrutiny of the performance of Labour leader Sir Keir Starmer, who saw his party gain the smallest share of the vote (1.6%) in any by-election since the Second World War.
Labour came fourth in the by-election with just 622 votes - trailing the third-placed Green Party - and losing the party's deposit in the process.
Last month, Sir Keir saw his party lose the Hartlepool by-election to the Conservatives - a seat that had been under Labour's control since it was created in the 1970s.