Building a million new homes between Oxford and Cambridge would destroy an area of countryside larger than Birmingham, campaigners have claimed.

The National Infrastructure Commission has recommended the building of up to one million homes by 2050 as part of development proposals between Oxford, Milton Keynes and Cambridge, dubbed the “Oxford-Cambridge Arc”.

But the Campaign to Protect Rural England (CPRE) has said it would mean the loss of 67,000 acres (27,000 hectares) of greenfield farmland and woodland to development.

Analysis by the campaign group suggests there is capacity for just 50,000 houses on previously developed or brownfield land within the Arc.

There are some 230,000 homes currently proposed or being built across the swathe of land, so meeting the one million target would require a huge boost to building – most of which would be built on open countryside.

CPRE also said that just 2,200 “affordable” homes are being built each year in the area, despite local authorities identifying a need for 12,000 such properties in the region.

A commitment to significant investment in improving the landscape, delivering sustainable transport and building affordable homes is “essential” if the recommendation is accepted by the Government in a move expected on Monday.

And the campaigners are calling for public debate about whether spending huge sums on developing the area is right, given that it is already attractive to employers and has a buoyant housing market.

The £3.5 billion earmarked for the “expressway” that will link the cities would be better spent on improving public transport in the region and restoring the East West Rail line, rather than locking in carbon emissions, pollution and car-dependency, they urged.

Paul Miner, from CPRE, said: ‘If given the green light, this development will change the face of England’s countryside forever.

“Whilst there will be a need for genuine affordable housing to meet local need in the area, the scale of these proposals is completely unacceptable.”

He said no formal environmental assessment or public consultation had taken place around developing the Arc, although it will cost at least £5.5 billion in public money.

He called for a strategic environmental assessment on the plans.

“The assessment must look at the impacts of both the proposed housing and transport development on the countryside, people’s health and well-being, and climate change in a holistic manner.

“Critically, we need much stronger commitments to protecting and improving the unique and precious rural landscapes in the Arc.”

The call from the CPRE comes after a study which warned residents in new housing developments are often forced to travel by car because of a lack of connections for cycling, walking or public transport.

The report by Transport For New Homes said a focus on meeting targets for new homes is coming at the expense of meeting residents’ needs, with developments built away from jobs, shops, services and good public transport.

A spokesman for the National Infrastructure Commission said: “Our recommendations come with the clear condition that new schemes should not compromise the high quality natural environment for existing and future residents, and do not need to involve any changes to existing Green Belt protections.

“In fact, our report made clear the need for significant investment in landscape improvements, affordable housing and sustainable transport.

“These changes are vital to make the most of the area’s economic potential and the contribution it makes to the wider UK economy.”

Communities Secretary James Brokenshire said: “Building the homes our country needs does not mean tearing up vast tracts of our countryside.

“The Oxford-Cambridge Arc is an opportunity to further strengthen rural economies, enhance the environment and benefit local communities.

“Our new planning rulebook strengthens protections for the Green Belt and we have set out our ambition to ensure housing projects are more environmentally friendly.”