The Campaign to Protect Rural England said 150,000 homes and 1,000 hectares of mines and commercial premises are being proposed for the areas, up from 81,000 homes planned in August 2012.
The group said the figures raised serious concerns about whether ministers were keeping their promises to safeguard the green belt.
CPRE 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."
He said green belts prevented urban sprawl and were the "green lungs" of British cities.
"Ministers now need to go further. Hard decisions are needed to help ensure both urban regeneration and protection of the green belt."
Housing Minister Mark Prisk assured people theat the government will maintain green belt land.