The Metropolitan Green Belt is a statutory green belt around London. It includes designated parts of Greater London and the surrounding counties of Bedfordshire, Berkshire, Buckinghamshire, Essex, Hertfordshire, Kent and Surrey in the South East and East of England regions.
The Metropolitan Green Belt was the first such area of protected open land to be mooted in the United Kingdom, by the Greater London Regional Planning Committee in 1935. The Green Belt (London and Home Counties) Act 1938 permitted local authorities around London to purchase land to be protected as open space and enter into covenants with landowners that open spaces would not be given over to development. The Town and Country Planning Act 1947 permitted local authorities to designate areas to be protected as part of the green belt within their development plans.
More information, including localities, on the Wikipedia page