Eight in 10 children aged 11 to 17 are not doing enough exercise and are at risk of a worldwide obesity crisis, a report is warning.
The World Health Organisation-led study is based on data gathered from 1.6 million people in 146 countries.
It found that more than 80 per cent of adolescents did not meet the WHO recommendation of at least an hour of physical activity a day.
And it warns that brain development and social skills are being damaged as well as children's health.
Fiona Bull, co-author of the report, urged policy makers worldwide to "act now for the health of this and future young generations".
Globally, the problems are worse overall for girls who are less active than boys typically.
The study found 85 per cent of girls and 78 per cent of boys failed to hit the exercise target overall.
The study looked at children between 2001 and 2016 and the authors say there's nothing to suggest the pattern has improved since then.
Leanne Riley, another co-author of the WHO report, said sedentary behaviour may be, in part, due to a rapid recent expansion in digital technology - meaning young people spend more time on phones, tablets and other screens than exercising.
"We've had this electronic revolution that seems to have changed adolescents' movement patterns and encourages them to sit more, to be less active, to drive more, walk less [and] be less active in general," she said.
In the United States, despite a campaign to promote physical exercise, obesity rates have risen among adolescents (especially those who eat food high in salt and sugar).
Riley added that "we need to do more if we want to halt the rise in obsesity.. and promote better rates of physical activity".
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Professor Russell Viner, the President of the Royal College of Paediatrics and Child Health, told the BBC the findings are "concerning".
"Children who are more active have better health and wellbeing and generally do better in school," he said.
"We should be making it easier for children and young people to have active and healthy lives - this is easier said than done."
