Government plans to build three million new homes by 2020 will create an "unsustainable urban sprawl", the Tories have warned.
The government promised 'robust' protection for green belts
Shadow planning minister Jacqui Lait said promises not to construct houses on green land were now "worthless".
The claim comes as a panel recommends the building of 32,000 new homes a year in south-east England.
But the government promised the green belt would continue to have "robust protections" from future developments.
An independent government-appointed panel has recommended the building of 32,000 new homes a year in south-east England - more than the 28,900 previously proposed by the Regional Assembly.
Shadow planning minister Jacqui Lait said: "Gordon Brown's empty promise that he would protect the green belt has been exposed to be worthless.
"His own government officials are planning to let rip with the concrete mixer and add to unsustainable urban sprawl."
She added: "Local residents will be powerless to stop the unelected bureaucrats building the sink estates of the 21st century."
The Financial Times newspaper reported that the Government Office for the South East had set a target of building up to 38,000 new houses annually, in line with last month's Housing Green Paper.
But the Department for Communities and Local Government denied it had set such a target and said the panel's review was based on government policy before Prime Minister Gordon Brown took office.
It added: "There will be no change to the robust protections of the green belt as the Housing Green Paper made clear.
"We believe it is possible to build the homes future generations need whilst protecting the environment and green spaces."