How many hours of television do you watch a day? New research suggests that parents who watch three hours of television a day could be affecting their children's TV viewing habits for the next three decades.

University College London found middle-aged "coach potatoes" were likely to have had the habit planted in them as young as 10.

The research warns parents that routinely switching off the TV and taking the family out for a walk or exercise can increase their child's chances of growing up to be a fit and healthy adult

Researchers compared the TV viewing habits of more than 6,000 British people at age 10 and age 42, using data from the Institute of Education's Centre for Longitudinal Studies.

Dr Mark Hamer, one of the UCL researchers, will tell a research conference today: "The problems that we have identified are not experienced exclusively by working-class families.

"However, parents from a lower socio-occupational class are more likely to be physically active at work and may compensate for this by spending more time sitting down during their leisure hours. Their children may then model their mothers' and fathers' leisure activity patterns."

Eighty-three per cent of the 1,546 cohort study members who reported watching more than three hours of TV at 42 had also watched TV "often" at age 10.

The study also showed that 42-year-olds who watched TV for at least three hours a day were more likely to be in only "fair" or "poor" health and to report that they were either overweight or obese.

The sons and daughters of manual workers were twice as likely as managers' children to watch more than three hours of TV a day at 42, even after their own educational qualifications had been taken into consideration.

Mr Harmer added: "This has important implications for policy and practice.

"It suggests that interventions to reduce passive TV viewing time should target children and their parents. Such initiatives could not only help today's children but help to reduce passive TV viewing in future generations.

"That could be extremely beneficial as research has also shown that TV viewing is associated with other health-risk behaviours, such as the consumption of energy-dense foods and cigarette smoking. Prolonged TV viewing has also been linked to type 2 diabetes and cardiovascular disease."

The research paper will be presented at the Centre for Longitudinal Study conference today.