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 per cent 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 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.”