There was a time when watching a group of children making a den in the woods was nothing unusual. Climbing trees or catching wasps in a jam jar was part of growing up. That was until Britain stopped going out to play.

Today, Britain's boys and girls are more likely than ever to spend their leisure time inside. It is a trend causing major headaches across society, with negative impacts on health and, studies suggest, the mental wellbeing of future generations.

A survey this month found that British children spend an average of five hours and 20 minutes in front of screens - TVs or computers - every day. That is 40 minutes more than five years ago, according to the market research agency Childwise, which carried out the survey of 1147 children in England, Scotland and Wales.

Computers and DVD players deliver unlimited play straight to the user while outdoor spaces, particularly in urban areas, are increasingly deemed unsafe places to go. Britain's children are getting fatter as a more sedentary lifestyle is embraced and their bond with the natural environment, once so treasured, is being broken.

That's why an innovative project adopted from Scandinavia is being viewed as a possible breakthrough in Scottish schools, helping draw children back to nature and away from the games console.

Forest schools were introduced to the UK after a team of students from Bridgwater College in Somerset travelled to Denmark to watch woodland-based sessions in practice. Studies from across the world have provided statistical evidence to prove that children are more physically active and motivated in outdoor environments. According to American research, children who play in natural environments undertake more creative, diverse and imaginative play than those who don't, enhancing their overall mental development.

What Forest Schools attempt to do is reconnect children, many in urban areas, with the green woodland spaces close to their doorsteps or classrooms. Already, teachers who have bought into the idea have reported positive results, ranging from higher levels of physical activity to calmer behaviour in lessons. The scheme has been rolled out further and there are now areas where older children, some with learning or behavioural issues, are also exposed to Forest School teaching.

The idea is very simple. A selected group attends a Forest School once every week or fortnight within school time. Qualified leaders run the sessions with support from teachers and assistants, and activities can be linked to the academic curriculum.

The type of teaching allows the children to explore their natural space but they learn quickly through hands-on experience rather than passively absorbing facts in a classroom. The participants are often urban children who may not have had regular access to green space. Tasks include tool-making, den-building, making fires and physical games using natural structures such as trees and boulders.

The idea is to build confidence, independence and togetherness by setting up small, achievable problem-solving tasks. The end result has been an increase in confidence and physical stamina among the children and an unpicking of the accepted wisdom that green spaces, particularly in cities, are dangerous places in which to play.

Edinburgh is now into the second year of a two-year project piloting Forest Schools at six city sites: Lauriston Castle grounds, Corstorphine Hill, Carberry Woods, Craigmillar Castle Park, Burdiehouse Burn Valley Park and Longstone Primary School grounds. The assessment from year one noted significant benefits from the programme. It has also had a knock-on effect in green space management and provision. Edinburgh City Council has recently undertaken its biggest upgrade of city parks for more than 40 years, aware of the growing benefit to children of being given access to clean, urban green space.

Last Friday, eight pupils from Liberton Primary were enjoying their second Forest School session at Burdiehouse Burn. Prior to a significant amount of replanting and regeneration by South Edinburgh Partnership, the park was known simply as the "dog toilet". Up the embankment from the river, the gables of tightly-packed modern houses are visible over railings. Behind where the P4 and P5 kids construct their dens in their waterproofs, industrial warehouses frame the scene.

However, significant work has been undertaken to give the woodland back to the community, making it a pleasant place to walk, exercise and learn about nature. Next week, the children are to work with the ranger who has been employed to man the woods, now a local nature reserve.

"I was determined we would use a local wood, not get on a minibus, because I wanted the children to come back in their own leisure time," says leader Ros Marshall, 43, who keeps a watchful eye on the children as they indulge in active "free play".

Marshall, who has degrees in education, botany and geography, was first captivated by Forest Schools after she saw footage from a school in a deprived area of Newcastle. "Because we cannot offer this to everyone, we generally target particular children who may have some kind of issues or difficulty. For them, it is wonderful in terms of confidence. They are leading the learning, which is very satisfying for them.

"A lot of it is to do with self-esteem. Some of the children have not experienced fantastic success in their classrooms so this is a fresh start, a different way of learning. It might suit their learning style better than being shut away in the classroom. Today, the boys have asked for more physical running-about games, while the girls have tended to come at it more quietly but are still getting a lot from it. One girl brought in a lantern, so she has obviously thought about it."

As the children huddle out of the rain for "snack time" in their self-made shelter, one boy tells me that one of his main hobbies is "playing on the computer". Some have been to the wood before, on bikes. Others haven't. What all of them have been doing, though, is being physically involved in the environment, barely noticing the rain that pours all around them.

Sally York is education policy advisor for the Forestry Commission Scotland, a key player in the Forest Education Initiative, the body behind the concept of Forest Schools. She has more than 15 years of teaching experience, much in the schools of inner-city London, and feels the idea of introducing "controlled risk" is vital for children's development. She also reasons that, if we are to get children back to doing healthy outdoor activities, we need to challenge some of the ideas we hold ourselves in our busy, pressurised, modern lives.

"What Forest Schools do is address people's perceptions of risk. A lot of kids are not getting allowed out to play any more because it is seen as risky.

"Driving children everywhere is seen as an acceptable risk but, statistically, the dangers are much greater than allowing children to play in woods with sticks or to be exposed to fire-making.

"What we are seeing, though, is that the children are gaining confidence because they are being given a degree of freedom outdoors to do things they would not normally be allowed to.

"If they are given freedom to explore and learn about risk in a controlled way, it will help them make choices and know about risk when they are older. It is the business of reconnecting children with the outdoors, connections which have been lost. Many of these children would never have been to these woodlands before. What we are now seeing is children leading their parents out to these green spaces at weekends to show them what they have been learning."

This is a step in the right direction in getting children - and parents - back out of the house and into natural settings. The idea is that this will lead to a long-term respect for the outdoors which will be passed on to future generations.

With obesity statistics demanding serious consideration and greater numbers of Scottish children showing signs of stress, there is an urgency to promote active learning programmes.

"Re-engaging people with natural environments has proven benefits physically, socially and psychologically," says Kevin Lafferty, Health Advisor with the Forestry Commission Scotland.

"Just walking in woodlands reduces stress and lowers blood pressure. Doing this regularly when young will have huge benefits into adulthood."

If Scotland wants to lose its "sick man of Europe" tag, getting back to nature might be the best place to start.