The number of houses planned for England's green belt has doubled in a year, a report discloses.
Plans now exist for more than 150,000 homes to be built on protected land, an analysis of council documents has found.
The sites include some of England's most scenic areas, including parts of Dorset and the rural outskirts of York.
In addition, more than 1,000 acres will be lost to office blocks, warehouses and the HS2 rail link, according to the research carried out by the Campaign to Protect Rural England (CPRE).
The increase comes after planning reforms diluted the protection given to the green belt and introduced a "presumption in favour of sustainable development".
The CPRE analysis shows that there are now dozens of areas of protected land where councils have given the go-ahead to development, including:
Widespread building on unspoilt land comes despite repeated promises by Coalition ministers that they would safeguard the green belt.
Approximately 12.4 per cent of England is designated as green-belt land, a status introduced in the Fifties to protect the countryside around major towns and cities, prevent urban sprawl and encourage the re-use of derelict urban land.
But in total, the CPRE found that 150,464 houses are planned for green-belt sites, compared with just over 81,000 when the same exercise was conducted in August last year.
The CPRE's report says that "swathes" of green-belt land in the Midlands and the North will be lost to HS2, including large sites near Manchester and Birmingham, to accommodate stations.
The proposed high-speed rail line between London and the North will also cut into green-belt land near Broxtowe in Nottinghamshire and Ruislip in north-west London.
Campaigners have suggested that Conservative ministers are at war over the building programme, with George Osborne, the Chancellor, at loggerheads with Eric Pickles, the Communities Secretary.
The extent of green space at risk of development is also causing growing alarm on the Tory back benches.
Shaun Spiers, the chief executive of the CPRE, said ministers are "deeply divided" over how much protection the green belt should have from development.
He said that Mr Pickles is fighting to preserve the countryside but Mr Osborne, regards the green belt as an "irritating impediment" to economic growth.
Nick Boles, the planning minister, has indicated that the large increase in Britain's population has made modest building on these areas unavoidable.
"The green belt is hard to defend at the best of times, given the development pressures it faces," said Mr Spiers.
"The job becomes almost impossible when powerful ministers such as the Chancellor and Nick Boles give out signals that it is up for grabs and that the need for growth trumps all other considerations."
Mr Spiers urged David Cameron to back Mr Pickles against the Chancellor.
"The only person who can sort this out is the Prime Minister," he added.
A source close to Mr Pickles said that it was "slightly fanciful" to suggest the Communities Secretary was "at war" with the Chancellor.
Councils are changing the boundaries of their green belt as part of their core strategy plans for development, under which they are obliged to set aside land for house building over the next five years.
The Planning Inspectorate, a quango which approves the local authorities' plans, has ordered some councils to review their green belt boundaries in order to meet these house-building targets.
Mark Prisk, the housing minister, said that the Government remained committed to maintaining the green belt.
"We have increased green belt protections by abolishing every single one of the last government's top-down regional strategies that sought to delete the green belt in 30 areas - Local Plans are now sovereign," he said.
Mr Prisk added that the Government is selling off brownfield land owned by the public sector for development and making it easier for empty buildings to be brought back into use.
However, the scale of building on green-belt land is causing increasing unrest on Tory back benches.
Five weeks ago, Chris Skidmore, a Conservative MP, set up an All-Party Parliamentary Group [APPG] to oppose development on green-belt land.
Twenty MPs have already joined the APPG including Cheryl Gillan, a former Cabinet minister, and a range of fellow Tories including Nigel Mills, Henry Smith, Gavin Williamson and Anne Main.
Anna Soubry, the outspoken health minister, has also challenged Mr Boles over plans to build on the green belt in her constituency.
In a letter to Mr Boles she wrote: "Assurances about localism and continuing protections for the green belt at ministerial level are flying in the face of advice from the inspectors, leaving local authorities with no alternative but to agree development on green-belt land."
Other Conservatives known to be concerned about the erosion of land with this protected status include Julian Sturdy, Guy Opperman, Andrew Jones and James Gray.
However, Mr Cameron is facing mounting pressure from Liberal Democrat ministers to build more housing. More than two million people are believed to be on the waiting list for social housing.
Andrew Mitchell, the former chief whip, has campaigned against proposals to build 10,000 homes on green-belt land in his Sutton Coldfield constituency.
If all the 150,000 properties mentioned in the CPRE's report were built, the lost green belt would amount to an area totalling 21.61 square miles.
The group said that there are enough brownfield sites to build 1.5 million homes and that it "should not be necessary" to sacrifice the countryside.
It is urging ministers to issue new guidance to ensure that losses of green-belt land are avoided or minimised.