Many children are heading for problems with obesity as a new study shows fewer than 3% of 11-year-olds are taking enough exercise.
Youngsters are advised to get an hour a day of moderate to vigorous activity, but experts writing in the Archives of Disease in Childhood found hardly any are doing so.
The researchers analysed a study on the health of 14,000 children, and noted that while the young people were twice as physically active as adults, they were still not engaging in enough exercise.
Boys were found to take more exercise than girls, and they were also more likely to do more sustained activities.
Study leader Professor Chris Riddoch, from the Sport and Exercise Science School for Health at the University of Bath, said: "It is a sobering thought that children's activity levels actually peak at around this age and decline precipitously during adolescence."
Tam Fry, from the National Obesity Forum, said: "The fact less than 3% of children are exercising does not surprise me.
"At the moment we have the aspiration of two hours of physical activity in schools per week, which is laughable.
"Children need one hour per day of physical activity which will make them sweat."