The startling figure was behind a new campaign by the charity for more park rangers and play supervisors to make parks safe for children.
After conducting an audit of almost 5,000 parks across the country earlier this month, the NSPCC found that 81 per cent of play areas in open spaces were unsupervised.
In response to increasing fears among parents about child abductions, paedophiles and drug abusers, the charity called for new measures to make parks more child- friendly. In particular, it called for more park rangers and playground supervisors who were vetted and visibly present in parks.
"We are not trying to exaggerate the risk. In general parks are relatively safe but there is a perceived danger by parents. They are worried and we want measures to reassure parents they are safe places," said Hilary Cross, the charity's spokeswoman.
Mothers and fathers are increasingly over-protective, fearing that their children could fall prey to abduction, assault or bullying, the charity found.
Eighty per cent of those surveyed in a Mori poll last year said they would not allow their children to play unsupervised in the park during the school holiday. A new consultation document by the Government's Social Exclusion Unit reported that there had been a gradual decline of park keepers providing informal surveillance over the past 20 years.
Only 23 per cent of parents described their youngsters as "outdoor children", whereas three times that number felt that was the right description in the Seventies.
Yesterday the NSPCC said it was working with the Local Government Association, the Institute of Leisure and Amenity Management and 40 local authorities to try to introduce increased security to reassure parents. The measures should also include child protection training for park rangers, more designated play areas, better lighting and visibility, and control of anti-social behaviour such as drug misuse.
Jim Harding, the charity's director, said: "Playing is not just fun for children. It improves their health, confidence and social skills.
"Through play, children develop vital skills which help protect them in the wider world. Parents do not want to limit their children's freedom by being over-protective but they do care for their safety.
"Supervised play areas help reassure them. The answer is not to keep the child out of the community but to make the community safe for the child."
Dr Ruth Henig, chairman of the Association of Police Authorities, said: "Police authorities will continue to work with the NSPCC in their efforts to protect children, particularly during the summer holidays."
Making Play Safe
The London borough of Newham has introduced a team of 18 park constables with some police powers throughout its parks.
At Thomas Coram Fields, in the London borough of Camden, adults are not let into a 7.5-acre play space unless with a child.
One of Stirling District Council's playgrounds incorporates equipment designed specifically for children with cerebral palsy. On-site carers and staffed play sessions are offered in summer.
The Mile End Project, in Tower Hamlets, London, plans to introduce rangers and closed-circuit TV to a child-friendly park that is currently being built. Swansea City Council employs police-vetted park rangers, trained in child protection.
The community in Hartlepool, with council support, set up the country's first Safe Parks Project. An increase in community events and activities was found to provide greater safety.
The Children's Play Council is promoting a Home Zones scheme across the country, where selected residential streets would have reduced speed limits.
Warrington Borough Council employs 20 vetted park rangers, identifiable in uniforms.