Physical inactivity among teenagers will cost the NHS billions as the youngsters grow older and suffer multiple health problems, a new report claims.
Lazy 11 to 15-year-olds will cost the health service an estimated £8.1 billion, according to the analysis.
The money will be spent treating chronic heart diseases, stroke, colon cancer and type 2 diabetes, according to the Centre for Economics and Business Research and charity StreetGames.
The document suggests that nearly half of children in this age bracket fail to achieve the recommended targets for exercise.
Jane Ashworth, chief executive of the sports charity StreetGames, said: "This report lays bare the economic and social cost we will pay if we don't get our young people moving.
"If we fail to address these issues it is akin to sitting on a time bomb.
"Helping young people in the most difficult circumstances take up a sporting habit for life is one of the most important things we can do."