Green belt land protecting countryside from urban sprawl is increasingly being threatened by government initiatives, rural campaigners have said.
The Campaign to Protect Rural England said green belt policy - introduced 50 years ago - faced a "major onslaught".
It said government proposals to build more homes and expand airports would have an impact on existing green belt and called for more to be created.
The government has said it is committed to Green Belt policy.
Plans to expand York University and widen the M6 in the West Midlands were also cited as future threats.
"We should be celebrating this 50th anniversary by creating new areas of Green Belt, rather than threatening existing ones," said CPRE chief executive Shaun Spiers.
Mr Spiers said the effectiveness of the policy "is being blunted by the top down imposition of boundary reviews and careless talk of 'replacing' lost Green Belt land elsewhere".
"A crucial element of Green Belts is the permanence of their boundaries," he added.
"It has been remarkably successful in stopping sprawl and protecting the countryside around our towns and cities, thereby improving the quality of life of us all."
The CPRE said developments that threatened the Green Belt across the country include:
Plans to expand Bath University, encompassing nearly 12 hectares of Green Belt land.
The Cambridgeshire Structure Plan Review Panel Report which recommends the removal of land from the county's Green Belt.
The building of about 6,000 houses west of Stevenage, Herts.
The construction of a fifth park and ride site outside Chester.
Construction of a sports venue serving Nottingham and Derby which will take up nearly 10 hectares of Green Belt.
Plans to build on thousands of homes in the Oxford Green Belt over the coming decades, despite opposition from the local county council.
The government's draft Vision for South East Dorset 2006 which includes an option to build thousands of homes.
Bradford's Unitary Development Plan which clears the way for development including housing.
The expansion of Newcastle Falcons Training Ground which takes 30 hectares out of the Green Belt around Tyne and Wear.
Proposals to either widen the M6 north of the West Midlands or build a new M6 toll road.
Plans by York University to expand its campus and build a business park which would impact on 117 hectares of Green Belt around York.