The Green Space Index by Fields in Trust found that more than 2.5 million people lived more than a 10-minute walk from the nearest area.
The charity has calculated that the average amount of green space per person is less than half of a six-yard-box on a football pitch.
A growing body of research links parks and green spaces to wellbeing.
“We actually found that 2.6 milion people did not live within a 10-minute walk of a green space,” explained Field in Trust policy and insight manager Alison McCann.
“In Great Britain, there is no statutory protection for green spaces,” she told BBC News.
“At the moment, we protect about 6% of our parks and green spaces, so we are calling for the Green Space Index to be used as an early warning system as we should be doing more to protect what we have got because of the multiple benefits they bring.”
Studies have shown that parks and urban green spaces deliver health and wellbeing benefits.
Researchers have calculated that outdoor exercise delivered an estimated £2.2bn of health benefits to adults in England each year.
The scientists calculated that more than eight million people each week took at least 30 minutes of “green exercise”.
In another study, carried out by a team from the European Centre for Environment and Human Health at the University of Exeter reported that living in an urban area with green spaces has a long-lasting positive impact on people’s mental well-being.
Ms McCann explained that research carried out by Fields in Trust found that parks delivered an estimated £34.2bn of health and wellbeing benefits each year.
“If you use parks regularly, you are reaping some huge rewards,” she said.
“But the Index is showing that us that not everyone has that close access, to parks, so they are potentially missing out on those benefits.”
Fields in Trust is launching a campaign on Monday to find the UK’s Best Park.
The award is open to all public green spaces, which can be nominated on the charity’s website.