Conservative MPs on Friday urged Boris Johnson to reconsider his “electorally toxic” planning reforms which they blamed for the dramatic by-election loss in one of the party’s traditional strongholds in southern England.

Voters’ fears over the UK prime minister’s liberalisation of planning laws were cited by Tory MPs and activists as the chief reason behind the defeat in the Chesham and Amersham by-election. The Buckinghamshire seat, in the Chilterns area of outstanding natural beauty, returned its first ever Liberal Democrat MP — Sarah Green — on Friday.

Soon after moving in to Number 10 in July 2019, Johnson promised to “transform the sclerotic planning system” in England. He has pledged to force all councils to rewrite their local development blueprints and deliver 300,000 new homes a year by 2025. Every local authority will have to designate land for either development or preservation.

Although the reforms are largely supported by Conservative MPs in the north and midlands of England, Johnson faces a mutiny from those in southern counties — including prominent figures such as his predecessor Theresa May. The rebellion, which is being organised by Theresa Villiers, the former environment secretary, is thought to include around 50 MPs.

The planning rebels are not only unhappy about the prospect of more housebuilding but also the proposed use of a computer algorithm to decide how many homes should be absorbed by each area. Although ministers have since had the algorithm tweaked to deliver a higher proportion of the new homes in urban areas, it has failed to neutralise the rebellion.

Bob Seely, the MP for the Isle of Wight who is against the proposed reforms, said the Chesham and Amersham by-election result showed that “a bad planning bill is now electorally toxic in Britain, therefore it is critical we get it right”.

Seely wants the algorithm re-examined and more consent given to individuals. “Instead of a ‘developers’ charter’, we need a community-led solution. Communities are happy with more development when you give them a say in what is happening in their area,” he said.

One longstanding Tory MP opposed to the reforms said: “We lost that seat because of planning and mine is on the line too. We need to fix the substance, but we also need a narrative. No one understands why we’re doing this.”

A Tory activist, who had canvassed in Chesham and Amersham said the government needed “to make the case for 300k homes a year, because development is unpopular with environmental issues so high in people’s consciousness”, adding that “until it persuades people they should accept thousands of homes, it is politically unviable”.

Johnson responded to the by-election loss by claiming there was “misunderstanding” about the reforms. “What we want is sensible plans to allow development on brownfield sites. We’re not going to build on greenbelt sites, we’re not going to build all over the countryside.”

In reality, however, the reforms will allow building in plenty of green fields, even though land designated as “greenbelt” will have some protections.

Matthew Spry, a director at planning consultancy Lichfields, said the “by-election result has put into sharp focus the difficulties the government would have if it tried to set housing requirement targets for every local authority in the country on a top-down basis”.

Simon Clarke, MP for Middlesbrough South and East Cleveland and a former government minister, who is in favour of the reforms said they were desperately needed.

“To fail to address this is to deny hundreds of thousands, probably millions of people the chance of home ownership, and to leave them trapped in a rental trap,” he said. “To govern, to be a party of government, is to make tough but responsible choices in the long-term interests of the country.”

The legislation is expected to arrive in the House of Commons in the autumn, with ministers prepared for a battle. “Planning reform is going to be the biggest challenge of the autumn. We absolutely have to push ahead with it, but politically it’s going to be very costly,” one cabinet minister admitted.

The rebels may struggle to defeat the government given that only MPs in England will be able to vote on the planning bill, which will exclude parties such as the Scottish National party, which with 45 MPs is the third largest in the UK parliament.

Robert Jenrick, the housing secretary leading the reforms, has been criticised by colleagues for the way he has engaged with concerned MPs.

One Tory representing a seat at threat from the Lib Dems said Jenrick was “handling colleagues with the subtlety of Ed Davey’s hitting the wall with a sledgehammer”, a reference to the Lib Dem leader’s post-election stunt celebrating the victory. “There is a complete lack of understanding about an issue that means for a lot of colleagues they don’t stay in parliament,” they added.

An ally of Jenrick said the housing secretary was holding frequent meetings to reassure concerned MPs. “All of these meetings have been friendly and constructive — to suggest otherwise is completely wrong.”

If Johnson is forced to water down the reforms he would not be the first British prime minister to retreat from ambitious plans for a shake-up of the planning system aimed at addressing the country’s housing shortage.

When the Tory-Lib Dem coalition led by David Cameron sought to do so, it ran into a huge backlash from the shires, led by the Daily Telegraph — at which point ministers put on the brakes.

Yet the widening gulf between homeowners and those struggling to get on the housing ladder — particularly young people — prompted Johnson’s government to return with fresh proposals designed to boost housing numbers.One senior Conservative said that the by-election showed the party needed to either own or bin the reform. “We’re going to have to shit or get off the pot: own it, and get in front of it. Or shut up about it. The way planning is dragging on and on is unsustainable. Boris has to lead on the issue, or it will eat him up.”