Official figures, used for drawing up planning policy, have failed to record the loss of 690 square miles of countryside since the end of the Second World War - an area larger than Greater London.
The Department of the Environment says 190 square miles a year were built on in England in the 1980s but the study, commissioned by the Council for the Protection of Rural England, puts this area at 460 square miles.
It condemns recent changes in Government countryside protection policies, saying they have been based on severely flawed statistics 'which led to this serious understatement of the rate of urbanisation'.
Between 1945 and 1990 the loss of rural land amounted to 2,720 square miles, an area larger than Greater London, Berkshire, Hertfordshire and Oxfordshire combined. The total urban area now stands at just under 15 per cent of England.
If fields and woods continue to turn into roads, industrial areas, housing and shopping developments as fast as they did during the 1980s then in 50 years 20 per cent of England will be urban. An area of countryside larger than Bristol is being lost each year.
The findings are in stark contrast to the Government's latest report on land use change, published last year. It concluded that by 2001 11 per cent of England will be in urban use; the CPRE's report says this percentage was exceeded long ago.
At the time the planning minister Sir George Young said: 'We often hear criticisms that our countryside is disappearing under a wave of development . . . this report refutes that claim.'
The CPRE's report was written by Geoffrey Sinclair, a consultant who has worked on land use issues for government bodies. He examined four sets of government data which give trends in changes in the way land is used and analysed them to give 'consensus' figures which he believes give the best estimates of countryside loss.
Until recently the Department of the Environment judged rates of urbanisation largely from the answers farmers gave on changes to their land for Ministry of Agriculture censuses. That neglected much woodland and countryside not on farms.
By the mid-1980s the Government had recognised that the census no longer provided reliable information about conversion to urban land, and the Department of the Environment began to monitor land use changes recorded by the Ordnance Survey.
Until 1987, one of the Government's main reasons for protecting the countryside was to keep it available for farming. Since then the policy has changed - rural land is to be preserved as much for its wildlife and scenic value, and limited development to provide jobs and houses is encouraged. The Government said at the time that rates of urbanisation had dropped to the lowest level since the 1920s.
Fiona Reynolds, CPRE's director, said: 'It raises the question of how long we can expect to make greater and greater demands of the soil, space and scenery that is England. This is one of the most fundamental issues of environmental sustainability.'
The Department of the Environment said it wanted time to study the report.
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