The president of the Campaign to Protect Rural England (CPRE) has warned the government is "nibbling hungrily" at England's green belt.
Campaigners are marking 50 years of green belt policy
Sir Max Hastings said he fears green fields will soon be concreted over as developers vie for what is left.
He told the CPRE annual meeting in London that the green belt is under increasing threat from building sprawl.
The government said it is committed to maintaining and increasing the green belt in all regions.
Sir Max said: "Speculators are selling plots within green belt land, holding out to buyers the prospect that a government which is already nibbling hungrily at the green belt in several areas will soon allow planning consents to be given in others.
"These circling sharks are disturbingly confident that Whitehall will throw them raw meat, because there is already blood on the water."
This summer, CPRE launched a campaign to protect these areas, 50 years after they became a nationwide planning policy.
The historian, columnist and former Fleet Street editor Sir Max said: "As green belt finds itself increasingly threatened by development pressures around major conurbations, we'll be highlighting its historic contribution, and seeking to emphasise its vital function in the future."
A spokesman for the Office of Deputy Prime Minister John Prescott said since 1997 the total area of green belt land in England has increased and more was proposed in emerging local plans.
He said there had been no relaxation of planning policy and there were plans to tighten it.
"Nationally, 67% of all new building is now on brownfield land, compared to 56% in 1997. Indeed, within designated green belt areas, 63% of development occurs on previously-developed brownfield sites, not on green fields," he said.
"This is the ultimate recycling policy - recycling land will help protect the green belt and enhance its quality rather than creating urban sprawl."