Children spend two-and-a-half months on average each year staring at screens, a study says.
Over 1m children in the UK are obese
The British Dietetic Association poll of 3,000 school children found a fifth of their time was spent playing video games, watching TV and using computers.
It also showed many ate meals in front of the TV and skipped breakfast to watch their favourite programmes.
Experts have said increasingly inactive lifestyles are one of the main causes of obesity.
Over 1m children in the UK are classed as obese and, if current treads continue, one in five boys and one in three girls will be obese by 2020.
The BDA poll of school-age children showed TV was the most popular screen activity among the teenagers with two hours a day spent watching programmes and two thirds watching at least 25 minutes before they even go to school.
The screen time also included work done on school computers - a third of children spent at least an hour doing this.
The survey also revealed that, instead of riding a bike to school, most children go on a bus or coach.
The survey showed dietary habits needed to improve as well.
More than a third of British teens eat their evening meal on their laps in front of the TV.
One in five skips breakfast - and almost 10% of those do so because they prefer to lie in bed.
Dr Frankie Phillips, registered dietician and BDA spokesperson, said: "Hi-tech gadgets have such a powerful influence on youth culture today and it's shocking how long kids spend in front of a screen each day.
"There are so many demands on the time and energies that teenagers have, with school and college work, part-time jobs and socialising.
"It has also amazed us that there is no real help or guidance provided for teens - they really are a forgotten generation."
She said the situation needed to change quickly.
"It doesn't have to be hard to get into healthy habits at all. The main key is just to move more to burn off the energy we're eating.
"Coupled with making time for regular meals, especially breakfast, reducing high-calorie foods and drinks and super-sizing fruit and vegetable portions, everyone can aim for a healthier size."
Dr Ian Campbell, medical director of Weight Concern, said the findings were "worrying, but not surprising".
"It is clear that computers and TV are very engaging and the challenge is to make physical activity engaging.
"It should not be a chore. Schools and parents need to encourage children to be more active"