The number of houses planned for green belt land has nearly doubled over the past year, according to research by the Campaign to Protect Rural England.
It said some 150,000 homes are being proposed for the areas - open spaces protected from development - compared with 81,000 homes planned in 2012.
Recent reforms have made it harder for councils to reject development plans.
The CPRE said the green belt was under threat but housing minister Mark Prisk said protection was being maintained.
He said the green belt "provides a valuable role in protecting against urban sprawl and providing a green lung around our towns and cities".
The green belt is intended to provide countryside access for urban dwellers and ensure conservation of nature, as well as maintaining agriculture and forestry.
England has 14 green belts, making up 13% of total land.
Planning laws in England were changed last year with the aim of speeding up decisions and boosting house building.
Under the National Planning Policy Framework, 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."
But this has caused a row with conservationists who have argued that it is more difficult to oppose development.
The government has consistently said the green belt would be protected under the reforms and rules around building on such land would be strengthened.
However, the CPRE said its research raised serious concerns about whether ministers were keeping their promises to safeguard the green belt.
Spokesman Paul Miner said: "The extent to which the threat is growing - nearly doubling in a year - is deeply worrying.
"It should not be necessary to build on green belt land when there is enough brownfield land available for a million and a half new homes.
"Ministers now need to go further. Hard decisions are needed to help ensure both urban regeneration and protection of the green belt'."
The housing minister said the coalition government had "increased green belt protections" by reversing decisions taken by the previous Labour government.
"We have also made it clear that the green belt is an inappropriate place for traveller sites and waste facilities, which will prevent speculative applications," said Mr Prisk.
"In addition the government is also selling off surplus public sector brownfield land and has extended permitted development rights on developed brownfield sites to help bring thousands of empty and underused buildings back into use," he added.
In recent months Labour has said it would reverse some of the coalition's planning changes in England.