Studies are under way for new homes in the Milton Keynes Quadrangle which includes Bedford, Milton Keynes, Corby and Northampton, the London-Stansted/Cambridge M1 Corridor and Ashford in Kent.
The fourth area being considered under the proposals is the East Thames Gateway.
In July this year, Deputy Prime Minister John Prescott outlined plans to build up to 200,000 houses in the region in the next five to 10 years.
"
The sheer scale of greenfield development being contemplated here is shocking
"
Henry Oliver, CPRE head of planning
The Government says the rising divorce rate and increasing lifespans are fuelling demand for new homes around the capital.
The Council for the Protection of Rural England (CPRE) says there is an urgent need for effective ways of constraining urban growth in the South East and promoting more balanced development across the regions.
'Shocking scale'
The CPRE voiced its concerns in a report on the future of Ashford.
It described the plan as "well beyond the capacity of the Kentish countryside to absorb without leaving permanent scars that we will live to regret".
Henry Oliver, CPRE head of planning, said: "The sheer scale of greenfield development being contemplated here is shocking."
The CPRE hopes to amend the government's Planning Bill, now in Parliament, which ministers say is needed to modernise and speed up the archaic planning system.