Watching two hours of television puts a millimetre on the waist of a child and reduces the ability to exercise, researchers have found.
Scientists who followed 1300 children from age 2 to 4 found that for every hour of extra TV a child watched each week, their waist measurement increased by 0.5mm and their jumping ability decreased by a third of a centimetre.
The average child watched 8.8 hours of television a week at the younger age and 14.8 hours when they got older. But one in six watched more than 18 hours a week, their parents told the Quebec Longitudinal Study of Child Development in Canada.
Experts recommend children over the age of 2 should watch no more than two hours of TV a day.
Dr Caroline Fitzpatrick, of the University of Montreal, who led the study, said: "The pursuit of sports by children depends in part on their perceived athletic competence. Behavioural dispositions can become entrenched during childhood as it is a critical period for the development of habits.
"Accordingly, the ability to perform well during childhood may promote participation in sporting activities in adulthood."
The leg strength measured by the standing long jump is crucial for sports such as football, skating and basketball, and loss of this strength is a less-known effect of watching TV.
Better known is that it leads to weight gain, as also revealed in the study, which showed a child who watches 18 hours of television at 4.5 years of age will by the age of 10 have an extra 7.6mm of waist.
The researchers stress they have demonstrated an association and further work is necessary to prove it is causal. But they say the findings should encourage authorities to act to reduce its ill-effects.
"The bottom line is that watching too much television - beyond the recommended amounts - is not good," co-author Dr Linda Pagani said. "These findings support clinical suspicions that more screen time in general contributes to the rise in excess weight in our population."
The study was published in the International Journal of Behavioural Nutrition and Physical Activity.