Government planning policy has put England's Green Belt at risk, a new survey has claimed.
The National Trust said half of the councils in England with Green Belt land were preparing to allocate some of it for development ahead of brownfield sites.
The charity said 51% of the councils it surveyed with green belts were now likely or very likely to allocate the land for development.
Overall, more than half of the 147 local authorities that responded to the survey said they had brownfield sites available that could help meet the five-year housing land supply target but these had not been considered viable.
The survey findings come 18 months after the Government put in place its National Planning Policy Framework, which aimed to speed up decisions and boost house building.
Local authorities are required to work out future housing needs in their area, and allocate sufficient land to meet it, with the "presumption in favour of sustainable development".
Earlier this year, the Campaign to Protect Rural England said the number of houses planned for Green Belt land has nearly doubled to 150,000 in the past 12 months.
Simon Jenkins, chairman of the National Trust, said: "The Green Belt has been the star feature of British town and country planning for half a century.
"In one of Europe's most congested countries, it has prevented urban sprawl, protected a vision of rural England and retained access to green spaces for urban dwellers that has been admired worldwide.
"Some councils may want to review their Green Belt boundaries as has always been possible, but the planning system as a whole should attach a greater weight to protecting green spaces.
"The Government's definition of 'sustainable' is in practice being interpreted as 'profitable', and has effectively killed the former planning presumption in favour of brownfield land.
"What is now happening is a policy of let rip, leading to steady erosion. For the first time in British planning history, planning control is now the slave not the master of profit."
Jonathan Carr-West, chief executive of the Local Government Information Unit, which carried out the survey, said: "This research shows that the National Planning Police Framework and targets around housing supply are putting significant strain on councils' ability to protect the Green Belt.
"It's crucial that we build more houses but we need to allow local authorities the flexibility to take a strategic view on how this should be managed locally."
The findings come as new National Planning Practice Guidance is to be issued by the Government by early next year, which the National Trust said could increase the threat to green spaces.
Ingrid Samuel, historic environment director at the National Trust, added: "What councils are saying is alarming.
"Green belt has historically been some of the country's most protected green space, and the National Planning Policy Framework was supposed to continue that protection.
"We need more homes and, if agreed in approved local plans, some of these may be built on previously undeveloped land, but the priority should remain brownfield first.
"The Prime Minister and Communities Secretary Eric Pickles have always made clear their desire to protect the Green Belt but this is not what the National Planning Policy Framework appears to be delivering on the ground.
"We are calling on the Government to amend its new guidance to ensure the planning system delivers on the Government's promise to deliver a 'brownfield first' policy, and to reaffirm its commitment to protect valued green spaces from development."
:: The Local Government Information Unit carried out a survey of local authorities in summer 2013. The LGIU received responses from 147 senior officers and local politicians responsible for public parks, green spaces and planning.
Of those, 59 respondents had Green Belt land in their local authority area and were asked the question: "Please rate the likelihood that your authority will allocate Green Belt land for development in the next five years".
There are 186 local authorities in England with Green Belt land.
Planning Minister Nick Boles said: "The green belt has a valuable role in protecting against urban sprawl and provides a green lung round our towns and cities.
"The coalition Government has safeguarded national green belt protection, abolished Labour's regional strategies which threatened to rip up the green belt, and introduced a new protection for valuable green spaces.
"The biggest threat to the green belt is Ed Miliband's plans to allow urban councils to dump development on their rural neighbours."