Teenagers who watch lots of television adverts eat far more junk food, researchers have warned.
Experts found youngsters who watched more than three hours of commercial TV a day ate more unhealthily than those who watched very little.
On average, they had ten extra snack items, such as chips, biscuits or fizzy drinks, a week totalling more than 500 a year.
However, when they watched TV without adverts, there was no link between screen time and the likelihood of eating more junk food, the Cancer Research UK team found, suggesting adverts may drive snacking.
Campaigners have long called for a ban on junk-food advertising before the 9pm watershed but UK ministers have so far resisted.
Advertising high-calorie food is banned during programmes aimed at children.
But health campaigners point out that this does not apply to mainstream shows such as The UK X Factor or live football matches, which millions of youngsters watch.
Britain's obesity problem is the worst in western Europe, with two-thirds of adults and a third of children overweight. Experts fear that the pattern will lead to major health problems, with rates of heart disease and diabetes predicted to soar.
Obesity is also the second biggest preventable cause of cancer after smoking.
Researcher Dr Jyotsna Vohra says: “This is the strongest evidence yet that junk food adverts could increase how much teens choose to eat.
“We're not claiming that every teenager who watches commercial TV will gorge on junk food, but this research suggests there is a strong association between advertisements and eating habits,” says Vohra.
She adds: “It's been ten years since the first, and only, TV junk food marketing regulations were introduced and they're seriously out of date. Our report suggests that reducing junk food TV marketing could help to halt the obesity crisis."
The study was based on a YouGov survey, which questioned 3,348 youngsters aged 11 to 19 on their TV viewing habits and diet.