Building on precious Westcountry land would "irreversibly damage the countryside" while failing to deliver much-needed economic growth, the Government has been warned.
Reports suggest that Chancellor George Osborne's "desperation" to kick-start the economy could lead to protected land being redesignated and given up to new housing.
But critics warned that the move was "ill thought through", would only damage the landscape and would not solve the country's economic woes.
Opposition was led by the former poet laureate Sir Andrew Motion, president of the Campaign to Protect Rural England (CPRE).
"It is unfortunate if some within the Government feel that undermining the green belt will help deliver economic growth," he said. "There is little evidence to support this, but plenty that it will irreversibly damage the countryside."
Sir Andrew said the green belt, which was designated to prevent towns and cities sprawling into the countryside, had been "hugely popular".
He added: "Now is the time for all those who care for our green belt, and indeed for the countryside as a whole, to stand up for it, including the many MPs who have assured their electorates that the green belt is safe in their hands."
Regional planning strategies, which included house building targets, were scrapped by the Government in favour of giving individual councils more control to meet local needs.
The Government said the Localism Act would "protect communities and the environment from top down pressure to build on the green belt".
Alison Seabeck, Labour MP for Plymouth Moor View, said it was "interesting that the right and left hand of Government were not in agreement".
"We all want to see economic recovery but that does not equate to building all over the green belt," she said yesterday.
"I think there are enormous tensions in Government over this and I can well understand by communities, whether in town or cities or in rural areas, would be worried about a planning free for all.
"When it has happened in the past, we have seen long ribbon developments and large housing estates, which have not been thought through and are not the right houses in the right place."
She added: "I just think George Osborne is desperate to find the means and mechanisms to make his economic policy work. It's ill thought through like so much of Government policy."
Torbay Liberal Democrat MP Adrian Sanders said the policy would not solve problems in the economy or the housing market.
"The issue of house building in not simply building more houses but building the right kind of houses to meet needs," he said.
"We have lots and lots of privately owned homes which are for sale that no-one can afford to buy and yet we have tens of thousand of people on waiting lists for rental accommodation who can't find appropriate properties.
"Those issues cannot be resolved by building more houses in the countryside and we should continue to protect the green belt."
New national planning policies require local authorities to allocate more than five years' worth of building land for new housing.
But the CPRE said it appeared that Government planning inspectors were putting pressure on local authorities to allow building in the green belt to meet the requirements.
The campaign group said it was "vital" the Government focused on "smart growth", prioritising investment and development within existing urban areas.
Tony Hilton, acting chairman of CPRE Cornwall, said urban sprawl would damage the region's most prized asset – its landscape.
"Green belt was created years ago to protect land from ever growing communities," Mr Hilton said.
"If we start building on land surrounding communities, people could drive across the Tamar Bridge and not even know they were in Cornwall.
"Because we are reliant on the tourist trade, it makes protecting the landscape very, very important."
A Treasury spokesman said: "The Government constantly considers a wide range of measures that can contribute to its key priority of delivering sustainable and balanced growth to understand their impact, and will announce initiatives when decisions have been taken."