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Boris Johnson was reeling from a shock by-election defeat amid warnings the Tories' “Blue Wall” is crumbling.

The Lib Dems pulled off a stunning victory on Thursday and overturned an 16,000 majority in Chesham and Amersham to seize the Conservative stronghold in Bucks.

Candidate Sarah Green won by 8,028 votes in a bombshell result which will trigger questions for the Conservatives.

Lib Dem leader Sir Ed Davey, who visits the constituency this morning, warned the Tory leader that if the result were repeated across the south "dozens of Conservative seats will fall to the Liberal Democrats" at the next election.

He told BBC Radio 4: "There is a lot more dissatisfaction with Boris Johnson than the polls suggest."

It comes amid reports Tory MPs with Lib Dems as their nearest rival are preparing to join up and pressure the PM not to take their constituencies for granted.

Sir Ed said: "This is a huge victory for the Liberal Democrats. The people of Chesham and Amersham have sent a shockwave through British politics.

(Image: Getty Images)

"We were told it was impossible for any party to beat the Tories here in Buckinghamshire.

"We were told this seat was too safe and the Tories too strong.

"This Liberal Democrat win has proved them utterly wrong. Across the South, the Tory Blue Wall is beginning to crumble. Here and in great swathes of the country, only the Liberal Democrats can beat the Conservatives and breach their Blue Wall.

"Sarah Green will be a great local champion for the people of Chesham and Amersham, and the Liberal Democrats will be a strong voice for everyone who feels let down and taken for granted by this dreadful Tory Government.

"This amazing win will strengthen our fight to safeguard our precious natural environment and build a fairer, greener, more caring country."

The ballot was triggered by the death of long-serving Tory Dame Cheryl Gillan in April. The seat had been Conservative ever since its creation in 1974.

But Lib Dems always believed they could snatch victory in the poll, campaigning on planning issues and opposing the HS2 railway which will carve through the constituency.

Why it happened

The result will worry Conservative strategists after the Prime Minister won the December 2019 general election by demolishing Labour's Red Wall across the North and Midlands.

The Lib Dems fought a fierce ground campaign and put the government's controversial planning reforms and being against HS2 at the centre of their campaign.

Labour's vote dramatically collapsed, with Keir Starmer's party picking up just 622 votes (1.6%) compared with 7,166 (12.9%) in 2019.

True-blue shire Tories have been warning the party was neglecting its traditional voters, however.

People in Chesham and Amersham backed Remain in the 2016 referendum and the result puts pressure on the PM, who won an 80-seat majority in 2019 pledging to "get Brexit done" not to focus too heavily on his coalition of Leave voters.

Such a resounding victory for a rival party will also be seen as an early verdict on the government's handling of the Covid pandemic.

What it means for Boris Johnson

The news is the first major electoral dent to the Prime Minister's authority since December 2019 and underlines the challenge he faces to repeat that victory at the next election.

Policing minister Kit Malthouse called the result “very disappointing” and said that 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.”

It means that the PM's policy agenda cannot focus on Red Wall Brexit voters to the exclusion of the party's traditional voters in areas such as the leafy Home Counties.

The results in full:

Sarah Green (LD) 21,517 (56.69%)

David Fleet (C) 13,489 (35.54%)

Carolyne Culver (Green) 1,480 (3.90%)

Natasa Pantelic (Lab) 622 (1.64%)

Alex Wilson (Reform) 414 (1.09%)

Carla Gregory (Breakthrough) 197 (0.52%)

Adrian Oliver (FA) 134 (0.35%)

Brendan Donnelly (Rejoin) 101 (0.27%)

LD maj 8,028 (21.15%)

Electorate 72,828; Turnout 37,954 (52.11%