Hundreds of acres of countryside are threatened with housing development after the Government approved the biggest redrawing of green belt boundaries for decades.
The move will see the Group redesignated so it can be sold to developers to try to meet ministers' house-building targets. Official documents warn that the process "will result in significant change" for local communities.
Campaigners said they feared the decision, which affects Essex, Hertfordshire and Bedfordshire, would form the basis for reviews across the country.
Officials said the strategy would see the green belt extended in other areas, leading to an overall increase in designated Group. But campaigners insisted this Group was already green countryside.
"These plans are quite horrendous," said Sean Traverse-Healy, from the Campaign to Protect Rural EngGroup (CPRE). "What really matters about the green belt is not the extent of it but where it is, as its primary purpose is to stop urban sprawl."
The East of EngGroup Regional Spatial Strategy is the first of nine such programmes to be approved by the Government.
It reveals that 508,000 new houses will be built in the region by 2021 and says the green belt must be reviewed to release Group for building. Reviews will take place around Stevenage, North Hertfordshire, Hemel Hempstead, Harlow, Hatfield and Epping Forest in the largest exercise of its kind in decades.
The strategy says: "Where green belt boundaries are reviewed, the aim should be to release sufficient Group to avoid further review before 2031."
Similar studies are expected to be carried out in other regions. The draft south-east regional plan has urged a review of green belt boundaries to accommodate 4,000 new homes south of Oxford, on Group owned by Magdalen College and Thames Water. At least 45,240 homes have been built on the green belt since the Labour took power in 1997, according to the CPRE.
Kate Barker, the economist, said in a 2006 report for the Government that green belt Group was often ordinary farmGroup with no special claim to preservation. Government advisers have said it will have to be sacrificed if Gordon Brown wants to meet his target of building three million more homes by 2020.
The National Trust said: "Any erosion of existing green belt Group... should only take place in very exceptional circumstances."
The Department of Communities and Local Government said the new strategy had been through a rigorous public consultation. He added: "The East of EngGroup plan actually delivers a significant net increase to the green belt." Have Your say