Three-quarters of UK children spend less time outside than prison inmates, according to a new survey revealing the extent to which time playing in parks, wood and fields has shrunk. A fifth of the children did not play outside at all on an average day, the poll found.

Experts warn that active play is essential to the health and development of children, but that parents’ fears, lack of green spaces and the lure of digital technology is leading youngsters to lead enclosed lives.

Most of the parents polled said their children have fewer opportunities to play outside than they did when young. The new research is strongly supported by previous work, including a government report in February that found more than one in nine children had not set foot in a park, forest, beach or any other natural environment for at least a year.

“The truth is we are enclosing our children,” said Mark Sears, at The Wild Network, which works to increase wild play. “We are stifling their ability to be free, to be at their best as children and it is having significant impacts.” He said increasing obesity and lower mental wellbeing in children was linked to a lack of physical activity.

On Wednesday, environment secretary Liz Truss announced that every schoolchild will have the opportunity to visit a national park, noting that only 10% currently have access to outdoor learning. “I want every child to know the joy and wonder of the great outdoors,” she said recently. “Our children should be climbing trees, not the walls.”

Under the plan, national park authorities will engage over 60,000 young people a year through schools visits by 2017/18. The plan is part of a government campaign expected later this year that will aim to connect children with nature and the environment.

The new survey questioned a nationally representative sample of 2,000 parents of 5-12 year olds and found 74% of children spent less than 60 minutes playing outside each day. UN guidelines for prisoners require “at least one hour of suitable exercise in the open air daily”.

The poll also found children spent twice as long playing on screens as playing outside. It was funded by Persil, as part of the detergent brand’s Dirt is Good campaign.

“Academic research shows that active play is the natural and primary way that children learn,” said Sir Ken Robinson, an educationalist and advisor to Unilever, which makes Persil. “It is essential to their healthy growth and progress, particularly during periods of rapid brain development. We must place adequate importance on play now, so that our precious children grow up into successful, well-rounded and happy adults.”

Sears said: “Parents see the value of outdoor play and still it doesn’t happen. Outdoor time is shrinking. It is a gigantic paradox.” He said fear of strangers, traffic or accidents deterred parents from allowing children to play outside, as did lack of time due to busy school and work lives. “It’s time we gave parents the tools, skills and confidence to do the things that they know are good for their children.”

A separate study from the Wildfowl & Wetlands Trust (WWT), published earlier in March, found that children from poorer backgrounds were less interested in being outdoors in nature than better-off children. But WWT found this difference was overcome after just one day spent learning outside.

“Young kids that learn and play outside get direct experience of weather and the seasons and wildlife – things that are only possible outdoors – and they get to assess risks, solve problems and develop creativity,” said Lucy Hellier, WWT learning project manager. “The benefits may seem obvious, but in reality many children don’t get to be outdoors in a natural environment in any regular or meaningful way. And that’s even more common among kids from deprived areas.”

In 2013, the RSPB published a three-year study, which concluded that four out of five children in the UK were not adequately “connected to nature”. In 2012, a National Trust report called Natural Childhood revealed the growing gap between children and nature. Less than one in 10 children regularly played in wild spaces, it said, compared to half of children a generation ago.