A Government-commissioned report has warned that extra house building in the South-East could increase the risk of flooding and create intense pressure to develop the protected Green Belt.
The report's warnings were revealed on the eve of the publication of the housing Green Paper, which is expected to outline plans to expand England's housing stock - possibly including a commitment to 20,000 more social rented homes a year.
Housing minister Yvette Cooper named home-building as one of the Government's top priorities for the coming years, and warned it would be "shameful" to neglect the younger generation's need for affordable places to live.
Earlier this month, Prime Minister Gordon Brown set out his ambition for 3 million new homes by 2020 - 250,000 more than the Government's previous target.
Many of these properties are expected to be located in the South-East, where prices have rocketed in recent years, in part due to the shortage of homes to house workers drawn to London's booming economy.
But the report by planning experts Roger Tym & Partners highlighted a set of risks if building in the crowded region is increased beyond the current planned rate of 29,000 units a year.
The report, obtained by the Conservatives, warns that additional house-building has "the potential to increase the risk of flash flooding" and will have "a negative effect... on the objective to reduce the risk of flooding" in areas like south Hampshire, the Sussex coast, central Oxfordshire and the Thames Gateway in Kent.
It also said that increasing housing targets would require "more space for development and therefore will need to use a greater amount of greenfield land". Building could have "a negative effect on the character of the countryside", as well as driving up road congestion, train over-crowding and water shortages.
Shadow local government secretary Eric Pickles said: "Gordon Brown's empty promise that he would protect the Green Belt has been exposed as a sham. His own Government consultants are warning that Labour's building plans will lead to environmental destruction on a massive scale. We face the prospect of more flooding, transport misery, water shortages and concrete mixers covering the Green Belt. Quality of life will suffer.
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"We need to build more homes. Yet Labour's policies are only going to deliver sprawling housing estates and high-density tower blocks, without proper infrastructure. Gordon Brown is going to build the sink estates of the 21st century, and local residents will be powerless to stop the unelected bureaucrats of an unelected Prime Minister."
