The stark warning comes after over 300 teachers, psychologists and children's authors claimed that the erosion of "unstructured, loosely supervised" playtime is dangerously affecting young people's health.

Larne GP Brian Dunn says he sees the result of increasing inactivity among children in his working life.

He said: "There are problems caused by an absence of outside playtime and children being driven to school. Children generally don't walk to school any more and there's less exposure to physical games. It's also easier to opt out of them.

"Then when children go home they just watch TV and play computer games. This is going to cause ill health in the future. We already have big problems with obesity and there may be psychological problems as well. Playing games is part of a child's development. Sitting in isolation playing computer games and watching TV is stunting their social development as well."

The GP says parents need to do more to encourage children to play outdoors.

He said: "Future problems can be avoided through more exercise and socialising. And it's not just down to the schools. There's a big fear among parents that things are not as safe as they used to be. But I'm not sure there is much evidence to back that up. Parents do over-react and over-protect their children."

In a letter published earlier this week the experts, which include author Philip Pullman and the chief executive of the Children's Society, Bob Reitemeier, warn that "many features of modern life seem to have eroded children's play".

These include increased traffic, parental anxiety about "stranger danger " and a "test-driven school curriculum".

Dr Brian Gaffney, chief executive of the Health Promotion Agency for Northern Ireland (HPA), says children should exercise for at least an hour a day every day.

He said: "Children in Northern Ireland are becoming more unfit. We know that many children have unlimited access to computer games, DVDs and many have televisions in their bedrooms which all contribute to a serious decline in their amount of physical activity.

"It is vitally important for children to be physically active as it is not just good for their physical health but it can also help with increased academic performance, enhance their self-esteem and contribute to positive mental health. Sitting in a room with only a computer or TV screen for company is surely a very isolating experience and one that should not be encouraged on a daily basis."