The Campaign to Protect Rural England has raised the alarm about plans to build on about 10,000 acres of green belt land.
Thousands of new homes will go up on greenfield sites
CPRE research found the green spaces are earmarked for housing, along with other uses such as airport expansion, roads and business parks.
It described the plans as an "onslaught" against greenfield sites.
The government said the total amount of greenbelt land had grown over the past 10 years.
The CPRE names eight areas of England where draft plans exist for 19,150 houses and 6,500 acres of further development.
10,000 new dwellings have been earmarked for Welwyn Hatfield District by 2021.
These can only be accommodated in existing Green Belt, the group says.
Another 3,000 new homes have been proposed for green belt land in Dorset, it says.
The CPRE's policy director Neil Sindon called on the government to protect green spaces in its forthcoming Planning White Paper.
He said: "This is the largest onslaught on the green belt for decades.
"It sets the context for a White Paper which many fear will further undermine the ability of planners to contain urban sprawl."
A spokesman for the Department of Communities and Local Government said: "Our record over the last nine years shows we can increase building on brownfield land and protect the greenbelt.
"Since 1997 we've seen greenbelt land across the country grow by 66,000 acres to 4.1 million acres."
Among the would-be beneficiaries of the building work is the BP pension fund, which stands to make millions from house building on the green belt near Harlow, Essex.
A statement from the fund said the plans had political support from all parties in Harlow.
It said: "The BP Pension Fund's plans for an exemplary sustainable urban extension to Harlow will both meet housing need and act as a catalyst for the regeneration of the town."
The government is expected to publish its planning White Paper later in the year.