Huge expanses of green belt land in England could be built on to meet government housing targets, it has emerged, with guidelines saying that councils lacking enough brownfield sites will be expected to offer untouched plots for construction.

The proposals, set out in consultation documents for the revised planning rules across England, prompted condemnation from campaigners, who accused ministers of making misleading promises about protecting the green belt.

There is also concern that the new national planning policy framework (NPPF) waters down targets for affordable homes, with a previous stipulation that at least 10% of new homes would have to be affordable being scrapped.

When the draft NPPF was unveiled in July, part of a pledge to build 1.5m new homes over five years, it was announced that councils that failed to meet construction targets could be forced to use “grey belt” sites, low-quality areas of the green belt such as former car parks and petrol stations, and sites on the edges of towns and villages.

But within the consultation document for the NPPF, totalling 15 chapters and nearly 30,000 words, it says that if there is not enough grey belt land councils should be pushed into building on “higher-performing” green belt sites, which would include previously untouched land integral to the green belt’s purpose.

The document sets out “a sequential test to guide release” of green belt land, starting with previously used brownfield sites, then grey belt areas, “and finally to higher performing green belt sites where these can be made sustainable”.

The consultation does not set out what is sustainable beyond ruling out national parks and sites of special scientific interest, and saying that construction should not “fundamentally undermine the function of the green belt across the area”.

Campaigners warned that such a vague definition meant that councils in many areas, especially around London, could face pressure to build on large areas of pristine land, saying this went against Labour’s manifesto promise to preserve the green belt.

Richard Knox-Johnston, the chair of the London Green Belt Council, which represents dozens of local authorities and environmental groups, said ministers had been “totally misleading” in their portrayal of the plans.

“If we were really just talking about car parks and petrol stations it would face little or no opposition. But the planned devastation will cause widespread protest, even among Labour MPs who now represent most of the country’s green belt constituencies.”

Lizzie Bundred Woodward, the planning policy manager for the CPRE countryside charity, said there was a risk of legal challenges that could slow down housebuilding.

It was vital, she said, for any green belt developments to be considered cumulatively. “While it would be difficult to argue that one small development ‘undermined’ an entire green belt, the cumulative effect of releasing green belt sites to accommodate multiple developments is another question altogether,” she said.

“The government needs to think about the total impact as well as consider each proposed development separately.”

Some local CPRE groups have expressed notably greater alarm. Andy Smith from Surrey CPRE warned that “very much of the higher performing green belt land will disappear”.

There is already concern that the definition of grey belt sites is so loosely defined that this could also result in a mass of construction on the green belt.

One planning consultancy, Lichfields, has said that “very significant areas” are likely to be affected. Another leading firm, LandTech, has estimated that up to 26,000 acres, enough for more than 300,000 houses, could be covered by sites designated as grey belt.

Officials at the Ministry of Housing, Communities and Local Government (MHCLG) argue that the concerns are overplayed. They say local councils will not want to use land integral to a local green belt – and will face a backlash if they try.

Another issue that has emerged from the new NPPF is an apparent lack of clarity on affordable homes. The previous NPPF set out a minimum of 10% affordable homes for new projects. This has been removed, with the only criteria being “that the mix of affordable housing required meets identified local needs”.

The MHCLG says the revised definition also takes in homes for affordable and social rent, while the 10% target referred only to affordable properties for purchase, but there are worries that developers will exploit the vagueness and build fewer affordable properties.

Tim Farron, the Liberal Democrats’ environment spokesperson, said: “We desperately need more homes built but these policies look set to meet demand rather than housing need.

“Removing affordability targets and letting developers off the hook with viability assessments is a betrayal of the millions of people across our country who desperately need a safe, good quality affordable home to live in.”

An MHCLG spokesperson said: “We do not recognise these claims. Our changes will make the outdated green belt rules fit for this century and councils, who know their communities best, will decide what is appropriate for development.

“Developers will be expected to meet affordability targets and if they can’t, they must provide strong evidence outlining why. The consultation remains open, and we will consider all responses.”