"We accept that there is a need for new housing, and we accept that some of it will have to be in the southeast," Nick Schoon, of the Council for the Protection of Rural England (CPRE), told BBC radio.

"But we have a general worry about more and more of the countryside disappearing under housing estates and concrete. Countryside is precious to all of us and it is disappearing."

Deputy Prime Minister John Prescott will unveil proposals to parliament later yesterday to build half a million new homes in England's southeast over the next 30 years.

The so-called "communities plan" has identified four target areas for expansion: Milton Keynes in Buckinghamshire, Ashford in Kent, Stansted in Essex and the Thames Gateway in east London.

Just 162,000 new homes were built in 2001 - the lowest figure for 75 years - and that has been blamed for spiralling housing costs.

The government hopes that building more houses will dampen costs and allow lower paid public sector workers to get a foothold on the property ladder.

Plans for transforming run-down communities in the north of England are also expected to be part of the package to be unveiled by Prescott.

Supporters of the government's development plans believe environmental campaigners are exaggerating the effects of the building on the countryside.

Professor Sir Peter Hall, professor of planning at University College London, said 89 percent of England was currently undeveloped.

"We are greatly exaggerating the scale of the problem," he told BBC radio. "It would take us 40 or 50 generations of building to completely destroy the countryside."