Sir Roger Gale told HuffPost UK the defeat was a “wake up call” for the prime minister amid a significant backbench rebellion over the plans, which MPs fear would concentrate house building in the party’s southern heartlands.
The Liberal Democrats took the Buckinghamshire seat for the first time in its history, winning a majority of 8,028 over the Tories on a stunning 25% swing.
The party focused heavily on the planning reforms in leaflets the seat, quoting Tory rebels including Theresa May attacking the policy.
The defeat has now intensified the rebellion, as MPs fear the pattern could be repeated across so-called “blue wall” southern traditional Tory seats.
Gale said that on Tory WhatsApp groups on Friday “the only thing anybody is talking about is planning”.
“The common theme is ‘me too, me too, me too’ - right across the south of England,” he told HuffPost UK.
“They are worried about the policy.
“But of course they’ve got an eye on their reelection chances.”
Johnson on Friday stressed that there has been “misunderstanding” about the reforms, insisting the government would not build on green belt land.
“What we want is sensible plans to allow development on brownfield sites,” he said.
“We’re not going to build on green belt sites, we’re not going to build all over the countryside.”
But Gale said the rebels were concerned about building on greenfield sites like agricultural land, stressing: “We’re not talking about green belt.”
The policy is in tatters
He backed calls for the government to instead focus on building houses on brownfield sites, empty commercial properties, and forcing developers to build on around a million unused planning consents.
On the upcoming planning bill, he said: “My personal view is: the policy is in tatters.
“They’ve got to wake up and smell the coffee.”
It came as leading rebel Bob Seely said the by-election result was “the start of a significant push-back from communities on planning”.
“Relentless housing targets - very often the wrong housing in the wrong areas - just feeds the hamster wheel of planning doom,” he told the BBC.
“We need a better way of doing things.
“We want to work with government to make this a success.
“But more of them same will be political suicide, and, as Winston Churchill said, the problem with political suicides is that you live to regret them.”
Johnson is facing fresh rebellion over planning after MPs on his own side last year effectively killed off a so-called “mutant algorithm”, which would have dramatically increased house-building in southern Tory cities and shires.
But the PM is believed to think home ownership is key to cementing the party’s gains in the so-called “red wall” in the north and Midlands and returned with fresh proposals to overhaul planning in last month’s Queen’s Speech.
The government has said it wants to speed up the planning process to deliver new homes and infrastructure more quickly, at the same time as protecting the environment, as part of efforts to hit Johnson’s target of building 300,000 homes a year by the mid-2020s.
But Tory rebels and countryside campaigners have warned that the bill will divide places into areas earmarked for either growth or protection, and that growth areas would undermine local democracy and give developers a green light to build on rural land.