The battle lines for one of Britain’s next political conflicts are being drawn in the rolling countryside bordering the South Downs National Park that stretches north towards London from the English Channel.

If housebuilding targets set by Boris Johnson’s government are to be met, and if landowners and developers prevail, swaths of this Sussex landscape of ancient woodland, farms and meandering chalk streams will be designated for the construction of new towns. It is a prospect that has local politicians, environmentalists and residents up in arms.

It has also placed the ruling Conservative party at odds with some of its traditional supporters, with last week’s by-election defeat in the Buckinghamshire constituency of Chesham and Amersham, just north of London, providing a taste of how political allegiances are under threat from what is built and where.

“It was a real shockwave,” said James MacCleary, leader of the district council in the East Sussex county town of Lewes, speaking about the result. The candidate for his party, the Liberal Democrats, overturned a hitherto unassailable Conservative majority with a record swing of 25 per cent.

“Concerns over the government’s attitude to rural areas and the protection of the countryside played a key role in that election. It’s obviously a warning shot for other Conservative seats,” he said, adding that in Lewes the Tories are defending a majority of less than 5 per cent.

In recent decades, greenfield land has mostly been protected. Building in East and West Sussex has tended instead to be in ribbons around existing towns.

Johnson has ambitions, however, to raise the number of homes built nationally to 300,000 a year, with a new formula to determine how these are distributed and an overhaul of planning laws to make it easier for developers to overcome opposition. Some 247,000 were built in the year to June 2019, the most for 33 years.

The new targets have ramped up pressure on local authorities; in the case of Lewes doubling the number of houses the council is supposed to plan for to 782 a year. Horsham council in West Sussex, where one of the more controversial new town proposals is next to a pioneering rewilding project at Knepp Castle, is under even greater pressure with an annual target of 1,200 homes.

“The core problem is that the government has set these arbitrary targets that are impossible to meet,” said Chris Smith, a retired care worker who publishes the Lewes Eye, a website scrutinising local governance.

He blamed the government for shifting goalposts, forcing councils with limited resources and time to review existing plans upwards in areas such as Lewes where there is limited land available outside the national park.

The prime minister’s “build build build” mantra is aimed at addressing the longstanding shortage of supply that partly underpins Britain’s housing crisis, and is part of his larger ambitions to tackle what he calls the country’s “great unresolved challenges”.

“We will . . . build back better, build back greener, build back faster and do that at the pace that this moment requires,” Johnson said last year.

On the ground, these plans appear at odds with other policies, notably on the environment, where his government has set out to make restoring biodiversity an integral part of local planning, with the goal of setting aside 30 per cent of all land for nature.

Meanwhile, the concentration of proposed new housing in the relatively prosperous south-east of the country, according to official statistics, raises issues for the government’s “levelling-up” programme for the north and midlands. This promise to redistribute resources from the south helped draw Labour supporters to the Conservatives, many for the first time, at the 2019 general election.

While construction costs are similar across Britain, profit margins vary markedly, making “levelling up” a tricky box to tick when it comes to building houses.

“Developers want to build all of the housing down here because it is where they make the money,” said MacCleary, whose council in Lewes is reviewing a proposal for a new town of 3,000 homes on 550 acres of farmland owned by Eton College, the prime minister’s alma mater.

The land was acquired from a distressed local farmer more than a decade ago and Eton’s initial proposal for a larger town there were deemed “not suitable, deliverable or developable” by planning authorities. But, given projected housing needs it was put forward for longer term consideration at a regional level.

The site at North Barnes Farm abuts the South Downs National Park, is crossed by a stream which is a spawning ground for a genetically distinct species of sea trout and is mostly reached by country lanes that residents fear will be overwhelmed.

In an ideal world, it would not be necessary to build in such areas, said a planning consultant working with one of the Sussex projects who asked not to be named. But given demand for housing, it is better to build, he argued, larger settlements that address climate change and social and economic needs for the 21st century than to continue with a piecemeal approach.

“I am all for levelling up but we can’t pretend we don’t need jobs and houses in the south east,” the consultant said.

He pointed out that many existing developments in Sussex were scattered, mostly serving professionals and commuters, and unreachable without a car. Some of the new proposals will be designed to be carbon neutral, with jobs you can walk and cycle to, and a high proportion of “affordable” housing. The protected designation of the South Downs, along with the High Weald and Surrey Hills further north, mean that sites where the scale to do this can be achieved are rare.

“I know what we are proposing will have major implications. I also know how thousands of people will benefit from beautiful homes built with local materials,” he said.

Welbeck, the land promotion company seeking planning permission on behalf of the Eton site, has outlined similar goals.

But local residents are sceptical. They fear that environmental and social aspects of these projects are designed to get them through planning, but will be jettisoned once granted, and landowners will flip land for handsome profits to home builders with less conscience.

“If you were trying to put together a sequence of events that would destroy our green and pleasant land, this would be it,” said Marc Munier, who has a family-run soap company near the Eton site and is heading Don’t Urbanise The Downs, a local campaign against it.

Pointing to the scale of developments already taking place in nearby commuter towns, he said [of the undeveloped countryside]: “Even people who don’t live in these areas like to know they exist.”

For local politicians, the choices are unpalatable, said one planning official. Accede to developers demands and you are likely to be voted out. Block them, and you risk having local plans overridden and then developers can apply for permission speculatively outside a more strategic local approach. “You are battling from both sides,” he said.