Researchers claim the "indoor generation" spends twice as much leisure time in the home as they do in green spaces as parents allow them to waste hours in front of the television, video games and the internet.
Experts said failure to play outside was fuelling obesity levels and leaving children less prepared to deal with the stresses of modern life.
Dr William Bird, GP and advisor to Natural England, said: "If you want to deal with obesity and physical activity, it has got to include the natural environment."
He added: "The outdoors for children creates resilience so they can cope with more stresses; they can actually increase their concentration, they can improve their work."
The comments were made in a study by researchers Future Foundation into the extent to which children and adults are becoming isolated to the outside world.
The report - based on separate surveys of 1,000 adults and 500 parents of children aged five to 16 - found that young people were increasingly likely to play indoors.
Parents reported that their children spent an average of 16 leisure hours indoors every week, compared with just eight hours outside.
Some 22 per cent of mothers and fathers said their children "rarely or never" spent any time outdoors, it was disclosed. Watching television was the most popular leisure activity for young people, the study said, followed by playing video games and using the internet.
Playing sport outdoors was only ranked as the eighth most popular way for children to spent their free time, said the study commissioned by Arla Foods.
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