A quarter of children spend less than half an hour outdoors each day, the National Trust has found.
More than half of youngsters aged seven to 12 spend less than an hour a day outside, while almost nine in 10 children have never taken part in traditional outdoor activities such as playing conkers or building a raft, according to research by the charity.
The National Trust said it had launched a campaign to "connect the cotton wool generation with nature" by getting 200,000 children playing outside this summer.
According to a poll of 1,000 parents and grandparents, 54% of children spend less than an hour outside each day, while one in four (25%) seven to 12-year-olds are outdoors for less than 30 minutes a day.
This compares to parents who spent an average of two hours and 34 minutes outside each day in their youth, the survey found.
More than half of grandparents (53%) spent more than three hours a day playing outside when they were aged seven to 12, compared to 6% of children today.
Despite this, 85% of those surveyed said playing outside was one of their greatest childhood memories, according to the poll conducted by Fly Research.
A second survey of almost 7,000 children aged seven to 12 found 92% had never built a raft and 87% had never played conkers.
Some 89% had not used a map or compass and 85% had not dammed a stream or explored a cave, research by the National Trust's Insights team found.
The charity's campaign, 5 0 things to do before you are 11-and-three-quarters, is staging more than 1,000 activities and events over the school holidays to encourage families to experience nature.
Outnumbered star Hugh Dennis, who is backing the campaign, said: "My most treasured childhood memories are of being in the outdoors so it's a sad thought that kids today aren't enjoying the experiences which we remember so fondly.
"There are so many simple delights to be had - and it doesn't have to be a chore or ordeal to do either.
"Back gardens can be as much as a treasure trove for kids as the seaside or a local park."
Helen Meech, assistant director for outdoors and engagement at the National Trust, said: "We really want kids to enjoy being in the outdoors and to care about nature, so it becomes part of their life as they grow up.
"The memories made as a child stay with you forever, and if outdoor places are part of these memories then hopefully children will grow up wanting to protect these special places for years to come.
"I'm sure if nature had a voice it would say that it misses today's children and wants to be part of their childhood adventures."