Boris Johnson warned ‘dozens of Conservative seats’ will fall after stunning Lib Dem by-election win

Anger over top-down planning shake-up seen as key reason for overturning of 16,000 majority in true-blue Chesham and Amersham

The Liberal Democrats have warned Boris Johnson they will snatch “dozens of Conservative seats” in the South if he keeps ignoring its voters, after the party’s stunning by-election win.

Ed Davey pointed to anger over top-down planning changes as a key reason for how a 16,000 majority in the true-blue Buckinghamshire constituency of Chesham and Amersham was overturned.

That shake-up – which critics say hands too much power to developers, undermining local democracy – may now be dead in the water, with the government fearing the backlash.

The Lib Dem leader said the controversy was “symbolic” of the belief that Southern voters are being ignored, as the Tories instead focus on knocking down Labour’s ‘Red Wall’ in the North.

“This will send a shockwave through British politics. It’s the Liberal Democrats’ best-ever by-election result and one of the very best ever,” Sir Ed said.

“And it will mean that, if repeated across the South, dozens of Conservative seats will fall to the Liberal Democrats.

“What was happening is people around here – and actually across the South as we found in the local elections – feel they’ve been ignored by the Conservatives, taken for granted.”

The planning changes would “give so much power to developers and take them away from communities and not result in the affordable housing people need”, he warned.

“But I think that was symbolic of the fact that the Conservatives are ignoring areas like this. Also, there’s actually a lot more dissatisfaction with Boris Johnson than the polls would suggest,” the Lib Dem leader added.

Kit Malthouse, the Home Office minister, admitted the defeat – on a 25 per cent swing, to hand victor Sarah Green an 8,000 majority in a seat held by the Tories since it was created in 1974 – was “disappointing”.

And he acknowledged the shift in the underlying electoral plates, saying: “We are seeing a change in British politics. We need to understand that.”

Sir Ed made no mention of lingering fury over the damage inflicted by Brexit, but one Tory MP who visited the seat said he was struck by “EU flags still fluttering” in some windows.

John Curtice, the elections guru, said the prime minister was losing ground among well-educated Southern voters – even as he won over working-class Leave supporters elsewhere.

And he predicted the result – in which Labour lost its deposit with just 622 votes, as anti-Tories voted tactically – would revive pressure for some form of joint working by the two parties.

“The speculation about that will continue and that is speculation that Keir Starmer will have to work out how to deal with,” Sir John said.