The Liberal Democrats have secured a shock victory in the Chesham and Amersham by-election, overturning a 16,000 majority in one of the safest Conservative seats in England.
In what southern Conservatives fear could be the first "chink in the blue wall" of affluent Tory seats, Lib Dem Sarah Green secured 56.7 per cent of the vote on Friday morning.
The Conservatives, who have held Buckinghamshire seat since it was created in 1974, finished in second place, trailing by 8,028 votes.
Her victory, which saw a 25.15 per cent swing to the Lib Dems, is one of the largest ever recorded in a by-election and comes on the back of mounting local opposition to HS2 and the Government’s controversial planning reforms.
It comes just one month after senior Tories told The Telegraph that Mr Johnson faces a "blue wall collapse" if he pressed ahead with the Planning Bill, due in the autumn, which will make it harder for local residents to object to new development.
Sir Ed Davey, the Lib Dem leader, described his party's Chesham and Amersham by-election victory as one of its "best ever" results, adding that "if repeated across the South dozens of Conservative seats will fall to the Liberal Democrats".
"I think what was happening is people around here and actually across the South, we found this in local elections, feel they've been ignored by the Conservatives, taken for granted, and a whole range of issues show that," he told BBC Radio 4’s Today programme.
Mr Davey cited proposed planning reforms as an example of a local issue which would not result in "the affordable housing people need".
He added: "There's actually a lot more dissatisfaction with Boris Johnson than the polls would suggest."
On Friday morning the Conservatives were scrambling to contain the fallout, with the local candidate Peter Fleet insisting that the Lib Dems had thrown the "kitchen sink... microwave, the table, the oven, the dishwasher, the dog, the cat, and anything else" at the campaign.
Others argued that the by-election allowed the Lib Dems to pour resources into the seat in a way they would be unable to in a general election, with the party also able to zone in on local grievances.
However, it is likely to send shockwaves across the Conservative Party, which since 2019 has focused its efforts on building on its gains in Labour’s traditional heartlands in the North and the Midlands.
While Boris Johnson’s policy agenda has been built around "levelling up" and delivering for working class and "left behind" communities, southern Tories have for months warned that the party has been neglecting its core vote.
A number of Conservative MPs have for years resisted the plans for HS2, the high speed rail line between London, Birmingham and major northern cities, due to significant local opposition in constituencies through which it is due to run.
But in recent months they have also pushed to overhaul the Government’s planning reforms, which would see the country split into areas designated for growth or preservation.
Ministers view the reforms as a vital part of the wider strategy to build on the Conservative gains across the Red Wall from the 2019 election, where high levels of home ownership are thought to have played a role in traditional Labour voters switching allegiances.
But in the South East and London, Conservative MPs fear that the zonal system will pave the way for swathes of new developments across leafy suburbs which form a core part of their electoral base.
Speaking in the wake of the by-election defeat, one MP told The Telegraph residual anti-Brexit sentiment and concern for HS2, as well as a change in demographics, was behind the upset.
However another blamed proposals to change the planning system.
"This has huge implications for Tory seats south of the Severn/Humber. Planning reforms are clearly going to have to have a rethink," the MP said. "HS2 and Brexit were sideshows. Both are done. This was a planning by-election and the Lib Dems really pushed the message on the reforms."
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 per cent of the vote.
The Green Party came third with 1,480 votes, with Labour trailing in fourth with just 622 votes, losing the party's deposit in the process.
In her acceptance speech, Ms Green said: "Tonight the voice of Chesham and Amersham is unmistakable. Together we have said, 'Enough is enough, we will be heard and this Government will listen'."