Campaigners are calling on local government and landowners to legally protect the parks and green spaces they own.
Fields in Trust say more than 2.5 million people in Britain do not have a park within 10 minutes walk of where they live.
Its research shows less than 6% of parks and green spaces are legally protected and are in danger of being sold or developed.
The charity says many people are missing the benefits of using parks and their loss would hit disadvantaged and under-represented communities hardest.
Its Green Space Index found that London, Yorkshire & the Humber and the North West and the North East all fall well below the minimum level of provision, while Scotland comes top.
Angela Lewis, head of programmes at Fields in Trust, said: 'It is concerning that millions of people across the country find themselves with not a single park or green space within a 10 minute walk.
'It is also worrying that many regions across the country are failing to meet even basic standards of provision.
'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.'