A government U-turn on plans to increase housebuilding in the Tory heartlands will make it harder to hit the target of building 300,000 new homes every year, leading planners have warned.

Robert Jenrick, the housing secretary, announced on Wednesday that 20 cities would instead be asked to build an extra 100,000 new homes in the next five years, heading off a rebellion from Conservative councillors and backbenchers.

But planners said it would make it more difficult to increase housebuilding because some cities were already struggling to find land on brownfield sites and it might trigger protracted negotiations where cities tried to hand off their housing requirements to neighbouring rural authorities.

Jenrick said the new housing plan was “levelling up” by targeting 35% increases in building in cities including Bradford, Hull, Leeds and Stoke. He said that after Covid’s economic impact there was an “opportunity to repurpose more commercial centres, offices and retail spaces into housing”.

Outside London, Birmingham is set to see the biggest increase in proposed building, with more than 1,200 additional new homes now expected on top of existing demand for 3,577 per year. Bristol is being told it will need to more than double its existing annual delivery from about 1,500 to 3,200, according to analysis by Lichfields, a planning and development consultancy.

But Matthew Spry, senior director of Lichfields, said the new system “makes it more difficult to deliver 300,000 homes a year”.

He said that some of the cities facing demands for more housebuilding had previously struggled because “they don’t have enough land”.

Toby Lloyd, Theresa May’s former housing adviser, said the strategy would not, on its own, boost housebuilding, and shifting targets to cities may make hitting housing targets harder, without other measures.

“The government remains stuck in the same hole as before, namely, how to square centralised target-setting with the desire to leave actual decisions on where homes go to local processes,” he said. “Shifting the balance of the formula towards cities and the north may relieve the immediate political pressure, but without stronger interventions as well it won’t address the deeper problems of planning, housing supply or affordability.”

Jenrick said: “This government wants to build more homes as a matter of social justice, for intergenerational fairness and to create jobs for working people. We are reforming our planning system to ensure it is simpler and more certain without compromising standards of design, quality and environmental protection.”

The shift to building in cities follows a political backlash in August against what one Tory MP described as a “mutant algorithm” which prioritised building in villages and towns in the south-east, while reducing construction of new housing in northern England. It was designed, in part, to suggest more housebuilding in areas of greatest unaffordability.

The Local Government Association said it would “seriously jeopardise” the government’s professed intent to level up economic activity in disadvantaged areas of the country. The CPRE charity said it would lead to “a massive loss of countryside”.

The original proposals would have reportedly required housebuilding in Newcastle to fall by 66%, Manchester by 37% and the north-east generally by 28%, while in the south-east outside London development would have risen by 57%.

Critics said the plans as they stood would abolish the traditional distinction in British planning between built-up areas and the 70-80% of land that was still rural and accelerate the decline of poorer cities.

The new plans include a £100m “brownfield land release fund” to promote urban regeneration and development on public sector land.

Ministers also allocated more than £67m in funding to the West Midlands and Greater Manchester authorities to deliver new homes.