Why did the LibDems win by-election – planning rules, HS2 or Brexit?

The Liberal Democrats’ leader Sir Ed Davey has claimed his party’s shock by-election win in Buckinghamshire has sent a “shockwave through British politics” – demonstrating that the blue wall of Tory seats in the south can be knocked down.

But just how significant is the by-election victory over the Conservatives in Chesham and Amersham? Does it herald a wider sea-change in attitudes, or was it merely a protest vote based on purely local issues?

Plans to build the new HS2 rail link – which cuts right through the areas’ Chiltern Hills – had provoked much local hostility, while the government’s recently announced proposals to reform planning laws have also riled voters in the constituency.

But the scale of the win was remarkable, suggesting something deeper and wider than an angry backlash against development may be going on.

Sarah Green secured a majority of 8,028 – overturning a comfortable Conservative majority of 16,000 in the seat which has been a Tory stronghold since its creation in 1974.

Polling guru Professor John Curtice pointed out that Chesham and Amersham is one of several Tory seats in the south where a majority voted to stay in the EU, with Brexit continuing to create a big cultural divide at each election.

He told BBC Radio 4’s Today programme that the Lib Dem victory was down to a “combination of the Remain inclination of the constituency and a number of local issues – HS2, planning”.

Prof Curtice added: “Labour voters, who of course the majority are Remain voters, were quite happy to switch to the Liberal Democrats.”

The elections expert also believes Boris Johnson could be losing ground among well-educated voters in the south – even as he wins over working-class Leave supporters in other parts of England.

Lib Dem leader Sir Ed Davey said his party’s “record-breaking” victory was part of a trend of beating the Tories in the south – pointing to recent gains in Oxfordshire, Cambridgeshire, Hertfordshire, Surrey and Wiltshire at the recent local elections.

“People feel that the Conservatives are out of touch, they’re not listening to places like this, they’re taking them for granted,” he told Today – predicting that the party could take “dozens” of Tory seats in the south.

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Mr Davey was challenged on how his party’s manifesto support for HS2 fitted with Ms Green promising voters she would be a “thorn in the side” of the rail line. “We still have problems with the way the HS2 company are going about their business – they have ignored local people.”

The government’s proposed planning reforms – which would see Whitehall earmark new areas of development on greenfield sites – sparked fears about excessive housebuilding in Chesham and Amersham’s countryside.

Will the Lib Dems be able to show voters in other Tory shires that they have a compelling alternative to top-down planning? Sir Ed claimed his party wanted to create a “community-led planning system” that created “affordable homes that are greener in the right places”.

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Labour finished an embarrassing fourth in the Buckinghamshire by-election, taking an astonishing low number of votes: 622. But Labour frontbencher Jess Phillips said the party could take some hope from the result.

She argued that there had been a “realignment” in the idea of “safe seats” after the Lib Dems victory in Chesham and Amersham, claiming local people “went for the most likely candidate to win”.

A Conservative Party source trotted out the “by-elections are always difficult for the governing party” line, while Tory chairwoman Amanda Milling said the “work starts now to show how it’s the Conservatives that can deliver on the people’s priorities and regain their support”.