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.