A study of 11,000 children, thought to be the largest of its kind conducted in Britain, has found that the risk of adult obesity increases by 7% for every additional hour of weekend television watched by five-year-olds.

Curiously the study, which was balanced for variations in social class, hereditary factors and birth weight, found no link between weekday television watching and obesity.

The authors conclude that weekend viewing has a much greater impact because it is more likely to take the place of outdoor play and other physical activity, helping to establish a sedentary lifestyle which persists into adulthood.

The ill effects were likely once viewing exceeded two hours a day at weekends.

“There is a clear [link] between television watching and risk of obesity,” said Russell Viner, a specialist from the paediatrics department at Middlesex hospital, London, who led the research which is published in the latest edition of The Journal of Pediatrics. “A lot more needs to be done by parents, but there is also good evidence that you can teach pre-school children not to watch television.”

The research tracked more than 11,000 people for 30 years from birth. While most of the group were watching less than two hours’ television a day by the age of five, some were watching four hours a day or more, and it was this group that tended to become overweight and obese in adulthood.

The group in the study were all born in April 1970 and grew up when there were just three television channels. Only ITV carried advertising and the junk food industry was in its infancy. This, say the authors, suggests it is the act of watching television that is the principal problem, not any influence that advertisements for junk food may have.

Of those who took part, 11.4% were obese by the age of 30. The researchers found that neither the number of days the child watched television after 6pm nor the type of programme watched strongly influenced the risk of obesity.

Many overweight adults regard patterns of behaviour picked up in childhood as the cause of their weight problems. Heather Dempsey, 30, a prize-winning slimmer from Glasgow, said that she used to watch three to four hours of television a night while eating chips or chocolate, and is anxious to prevent her children going the same way.

Joanna Chamberlain, 46, a website manager from Caterham, Surrey, said: “I was overweight from the age of eight. When I came home from school I would watch television from 4-7pm. It was what we did for entertainment.

“Two years ago I decided things needed to change. I started doing exercise and I hardly watch anything now.”

Hard evidence of a link between lack of physical activity, television watching and obesity comes at a time when many middle-class parents are already banning or restricting their children’s access to the box. Last week hundreds of people joined a protest in Liverpool against the media diet on offer to children.

It was the latest in a series of demonstrations organised by Mediamarch, a lobby group concerned about what it sees as television’s dominance in children’s lives.

Last week the pop star Madonna revealed that she does not let her children watch broadcast television, restricting them to occasional videos. “Television is poison,” said the singer who made her fortune out of mass media exposure.

Experts on obesity point out that television is used in schools and nurseries as a low-cost form of child-minding. “Some of them use TV when it is raining and the children can’t play outside, or just to keep toddlers quiet,” said Tim Lobstein, an adviser to the International Obesity TaskForce.

“They are signalling to parents that prolonged TV watching is acceptable.”

Others say adult obesity is a complex problem. Paul Gateley, professor of exercise physiology and health at Leeds Metropolitan University and who pioneered fat reduction camps for children, said: “Some children watch four or five hours every night, but you have to look at what they’re doing the rest of the time.”

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