Experts say this is leading to an increasing risk of ill health, with the vast majority of young people also failing to take the recommended level of exercise each day.
The study found a “continuous steep increase” between 2002 and 2014 in the proportion of children and young people using technology for two hours or more each weekday for things like social media, surfing the internet and homework.
In England in 2014, for children aged 11 to 15, 74.6 per cent of girls and 76.5 per cent of boys used a computer, tablet or phone for two or more hours on a weekday that was not for playing games.
The WHO, together with experts from the Health Behaviour in School-age Children study, sent questionnaires to more than 200,000 children in schools in 42 countries, including 5,335 in England, 5,932 in Scotland and 5,154 in Wales.
A breakdown by age showed children as young as 11 spending a large chunk of time online.
In Scotland, the figure was 42 per cent of boys and 45 per cent of girls, while in England it was 39 per cent of boys and 43 per cent of girls.
Out of 42 countries, Wales ranked third, Scotland was fourth and England was seventh.
When it came to using computers, tablets or smartphones just for games, between a third and two-thirds of children were spending two or more hours every weekday on them.
She said, “We know that a positive impact of social media is social connectedness and the sense of interaction.
“But we also know there are risks, such as cyber bullying and impact on mental health, as well as things like missing out on sleep.
“Also, there are longer-term impacts on physical health from being sedentary.”
She said these risks included cardiovascular disease, obesity and Type 2 diabetes.
“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.”
Tam Fry, chairman of the National Obesity Forum, said gadgets were taking their toll on the well-being of youngsters.
He said: ” Adolescents are now slaves to handheld devices and this is doing nothing for their health.
“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.”
Only about a fifth of boys in England, Wales and Scotland meet the Government recommendation of exercising for an hour a day.
The figure is worse for girls, with only 14 per cent in Scotland and 12 per cent in England and Wales.
Dr Inchley said: “We need to find ways to make young people more active.
“Maybe they are getting lifts in the car, or are not out playing in the streets any more in the way they used to.”
She said having two working parents could also affect exercise levels, with some children at after-school clubs instead of being outside.
Dr Steven Mann, ukactive research director, said: “Modern life has changed, but when teens are spending hours hunched over Facebook, Instagram and video games, they simply aren’t getting the exercise that they need.”