Despite commitments by the Government to preserve green belt land, it has been "seriously eroded", with an average of more than 1,100 hectares a year being developed between 1997 and 2003, the conservation charity said.

It said more than 45,000 homes - equivalent to a city the size of Bath - had been built on green belt since 1997.

The CPRE welcomed the move by the Government to create the New Forest National Park from existing green belt land and the refusal of damaging development proposals in the West Midlands.

But it warned that land designated to stop urban sprawl across England was under threat from developments including new homes, airport extensions, park-and-ride schemes and wind turbines.

Overall, green belt coverage has increased by more than 30,000 hectares since 1997 - if conversion of 47,300 hectares into the New Forest National Park is taken into account - and now accounts for 13% of land in England.

But the CPRE warns the policy is being weakened where it is most needed and most under pressure - at the edge of towns and cities.

The campaigners also accused Government planning inspectors of undermining protection of the green belt and suggesting it should not be treated as permanent.

Planning Minister Iain Wright said: "This analysis is flawed and one sided. The suggestion that the amount of green belt is falling overall is deeply misleading.

"What the CPRE fail to tell you is that since 1997 the overall amount of green belt has grown by 82,000 acres (33,000 hectares)."