Think tank the Social Market Foundation has warned that two million homes will have to be built on green belt sites to meet government plans to tackle the housing shortage.
Many believe the English countryside is under threat
But what is the English green belt and why does it exist?
What is the green belt?
Green belt is a planning tool, first introduced for London in 1938 but rolled out to England as a whole by a government circular in 1955.
It urged local councils to consider designating green belts where they wanted to restrict urban growth.
Green belts now cover 13% of England (around one-and-a-half million hectares). Wales has one green belt, between Cardiff and Newport, while Scotland has seven and Northern Ireland has 30 - each has its own policy guidance.
Green belt in England is protected both by normal planning controls and against "inappropriate development" within its boundaries.
Where is it?
There are 14 separate green belts in England, varying in size from 486,000 hectares around London to just 700 hectares at Burton-on-Trent.
What is the green belt for?
Green belt aims to stop urban sprawl and the merging of settlements, preserve the character of historic towns and encourage development to locate within existing built-up areas.
The quality or appearance of land is not a factor in its designation as green belt and it does not have to be "green".
Richard Bate, of planning consultancy Green Balance, says its primary function is to "encourage development to go to the places where it will do most good and to discourage it in places where it will do quite significant harm".
Is green belt different to a greenfield site?
Yes. A greenfield site is a piece of land that has not got development on it, while green belt has specific planning restrictions.
So, what is a brownfield site?
Brownfield land is another term for previously-developed land in rural and urban areas.
However, it does not include agricultural or forestry land or buildings.
Government policy calls on local planning authorities to maximise the use of previously-developed land.
Its target is for 60% of new housing to be provided on brownfield land.
When were green belts established?
The green belts around London, Birmingham and Sheffield were among the first to be established in the 1930s.
The first official proposal "to provide a reserve supply of public open spaces and of recreational areas and to establish a green belt or girdle of open space" was made by the Greater London Regional Planning Committee in 1935.
WHY HAVE A GREEN BELT?
Protect the countryside from urban sprawl
Encourage regeneration of sites within towns and cities
Prevent towns from merging into each other
Protect country setting of historic towns and cities
The Town and Country Planning Act 1947 then allowed local authorities to include green belt proposals in their development plans.
And in 1955, the government set out a green belt policy asking for local authorities to consider protecting any land acquired around their towns and cities "by the formal designation of clearly-defined green belts".
The aims of the policy were to prevent urban sprawl and protect the countryside from further encroachment.
Mr Bate said the pressure for major expansion of towns and cities in the post-war period led to the urgency to develop a policy on green belt land.
Is the green belt under threat?
Countryside campaigners have long accused local authorities and the government of allowing too much development in green belts, which they claim are being eroded.
The Campaign to Protect Rural England says green belts are being nibbled away at a rate of more than 800 hectares (1,977 acres) a year.
But the Department of Communities and Local Government says that since 1997, green belt has actually increased by 64,000 hectares (158,147 acres).
Mr Bate also believes the green belt is here to stay.
He said: "It has been going a long time and for almost as long as green belt has been going, there have been claims that green belt has been getting in the way of development and should be relaxed.
"But the fact that there are these claims that it should be relaxed is proving that it is doing its job."