Toddlers have as inactive a lifestyle as office workers, researchers have warned.
Researchers warn children are not active enough
University of Glasgow researchers found three-year-olds spend just 25 minutes a day exercising, half the recommended time.
They say if young children adopt sedentary habits they wil be more prone to obesity later in life.
Writing in The Lancet, they say parents must be educated about the importance of exercise for pre-school children.
Experts are increasingly worried about childhood obesity.
It is estimated around one in 12 six-year-olds and one in seven 15-year-olds are obese.
'Old before their time'
The Glasgow researchers assessed 78 children aged three, and then re-evaluated 72 of them when they were five.
They measured how active they were in two ways; using a device strapped to their wrists, and by measuring how long it took them to excrete an energy drink by testing their urine for a particular chemical.
They found that at both ages, children typically spent just 20 to 25 minutes a day in moderate to vigorous physical activity.
Dr John Reilly, of the Division of Developmental Medicine at the University of Glasgow who led the research, told BBC News Online: "There is a widespread perception among parents and health and educational professionals that young children are spontaneously active.
"But something we show is that children, well before they go to school, are as inactive as many office workers.
"They are old before their time."
He said the problem was often that children spent far more time watching television than they used to.
Dr Reilly added: "Public-health interventions are needed urgently, and these must involve population-based strategies that increase physical activity, reduce sedentary behaviour, or both in early life."
'Time to get serious'
In an editorial in the Lancet, James Hill From the Center for Human Nutrition at the University of Colorado Health Sciences Center, US, said: "It is time for the UK to take action to prevent the excessive weight gain that is likely to occur in its young children. Increasing physical activity must be a part of any national prevention efforts for weight gain.
"Changing behaviour to prevent weight gain will be easier than treating obesity once established.
"It is time to get serious about prevention of weight gain in the UK."