These findings show that there is an urgent need to ensure the current level of park and green space provision is maintained and review what more can be done to legally protect them for future generations to enjoy.
About 2.5 million people across Britain are more than a 10-minute walk from a park or green space, a charity has claimed.
Fields in Trust has developed a "green space index" using new ordnance survey data to map the availability of parks and recreation grounds for communities across the country.
It is warning large numbers of people who do not have easy access to parks nearby could miss out on the benefits they can provide for health and wellbeing, and it said the situation could get worse with public sector cuts.
Although Britain has some 216,160 hectares (534,000 acres) of publicly-accessible local parks and green spaces, Fields in Trust said just six percent of this is legally protected through the organisation.
With austerity leading to pressure on parks and green space provision, it said there is a risk that a lack of legal protection could lead to more areas being sold off or developed.
Disadvantaged communities are likely to be most affected by any future losses, it warned.
Fields in Trust, which was set up more than 90 years ago to champion open spaces and playing fields, is urging local authorities and landowners to take steps to legally protect the green areas they own.
The green space index also ranks regions and nations of Britain against its standard of what should be the minimum provision of parks, play facilities and outdoor sport per 1,000 people.
Scotland tops the index for green space provision and the largest amount that is protected, while London, Yorkshire and the Humber, the North West and North East of England all fall below the minimum level, Fields in Trust said.
The average amount of green space per person in Britain is just over 35 square metres, less than half the size of a six-yard box on a football pitch.
Angela Lewis, head of programmes at Fields in Trust, said: "Parks and green spaces are much-loved by people and provide many benefits to society.
"But our new green space index shows, for the first time, that there is insufficient access to parks and green spaces and that far too few are legally protected, leaving those unprotected vulnerable to loss or development.
"These findings show that there is an urgent need to ensure the current level of park and green space provision is maintained and review what more can be done to legally protect them for future generations to enjoy."
Fields in Trust plans to update the index on a regular basis to track changes in green space provision.
An Ministry of Housing, Communities and Local Government spokesman said: "Parks and green spaces are vital public assets that boost our health and wellbeing with natural spaces everyone can enjoy.
"That's why this government continues to work with experts in the Parks Action Group, including Fields in Trust, to help maximise the rewards they bring, and our revised national planning policy framework lets communities protect parks in their area."
Emily Beament is the Press Association environment correspondent.