Green belt protection must be strengthened, campaigners urged as they warned large scale developments were being approved on designated land around the country.
The Campaign to Protect Rural England (CPRE) said the threat of houses being planned and given the go-ahead on the green belt had increased since 2012 when new planning rules were brought in.
The number of major planning bids being approved on greenfield sites in the green belt - designed to protect open countryside around towns and cities and prevent urban sprawl - has risen from 15 to 27 a year since 2012, it said.
CPRE pointed to large developments which have been approved on green belt land recently, including Effingham, near Guildford, Surrey, where almost 300 homes have been approved by Housing Secretary Sajid Javid on appeal.
In Rochdale, Greater Manchester, a development in South Heywood will see around 1,000 homes built after approval by the local council, and a Ryder Cup golf course and more than 1,000 homes have been given the green light at Hulton Park, Bolton.
The Government has said 300,000 new homes need to be built each year in England to meet demand and help more people get on the housing ladder, but has also said it wants to maintain green belt protections.
But there are concerns that, of the hundreds of thousands of homes planned for the green belt, only a small proportion are "affordable", while many are more expensive "executive homes" which are profitable for developers.
CPRE's Paul Miner said: "It's completely unacceptable that developments of this scale are being approved on green belt land without consideration of brownfield alternatives.
"Despite its protected status, last year we saw almost half a million houses planned on green belt - 54% more than 2016.
"And only 16% of these were classed as 'affordable'."
The Government is considering changes to the national planning policy framework (NPPF), and CPRE has welcomed moves to ensure councils demonstrate they have considered brownfield sites and other options before altering green belt boundaries.
It has also backed the stronger protection proposed for national parks and areas of outstanding natural beauty (AONBs) against large developments.
But Mr Miner said: "Given that the proposed changes to planning rules, currently out for consultation, focus on meeting market demand, it is urgent that the Secretary of State makes clear that economic considerations do not, in and of themselves, mean we should build on Green Belt."
The charity wants a clear statement that high housing demand and aspirations of growth do not, in themselves, constitute a reason for altering green belt boundaries.
And areas with significant constraints to development should not have to plan for growth or meet unrealistically high housing targets, the charity said.