A “steep increase” in the use of computers, phones and other devices is putting young Britons’ health at risk, according to a World Health Organisation report.
Data for England, Wales and Scotland shows that more than three-quarters of children aged 11 to 15 spend two or more hours using electronic devices on weekdays.
Experts say this is reducing the amount of time children take to exercise each day, leading to an increased risk of ill health.
Tech use more than tripled for girls 15 and over between 2002 and 2014, with social media blamed for the increase.
Lead author Dr Jo Inchley, from the University of St Andrews, said risks posed by the increased use of social media included cyberbullying and sleep deprivation.
Inactivity also poses the long-term risk of cardiovascular disease, obesity and Type 2 diabetes, she added.
Dr Inchley said: “One of the main challenges for us is that this kind of activity (social media and computer use) is so much part of young people’s lives these days, how do we manage this and the health risks associated with it?
“It’s about reducing time being spent sedentary, and ensuring that children still have opportunity to be active.
“We really need to start addressing these challenges now.”
In England in 2014, 74.6% of girls and 76.5% of boys aged 11 to 15 used an electronic device for two or more hours on a weekday, according to the report.
In Scotland, the figure was 79.9% of girls and 83.6% of boys, while in Wales it was 76.4% of girls and 84.6% of boys.
Of the 42 countries studied by the WHO, Scotland came top for computer use by girls, while Wales came second for use by boys.
Tam Fry, chairman of the National Obesity Forum, said the report showed adolescents “are now slaves to handheld devices”.
He said: “Incredibly, teenagers believe that playing computer games with their friends from the privacy of their bedrooms is a form of physical activity and rebel if grounded from their Facebooks or Instagrams.”