Children should do at least 90 minutes exercise each day, experts say.
Exercise could be walking to school
The current UK guidelines recommend an hour of exercise - but a recent study found only one in 10 children of school age achieve that limit.
Writing in The Lancet, they say children should up their activity levels in order to ward off heart disease and obesity.
The Department of Health said it would consider whether its guidelines needed to be reviewed following the study.
If current trends continue, half of all children in England could be obese by 2020.
Among children, the rates of obesity have tripled during the last 20 years. One in 10 six-year-olds is obese.
The authors of the latest study stress that getting enough exercise is important not only to tackle the problem of childhood obesity, but also to prevent future generations dying prematurely from illnesses associated with sedentary lifestyles.
They looked at over 1,730 children, aged nine or 15 years, from schools in Denmark, Estonia, and Portugal.
For each child they measured a combination of risk factors for cardiovascular disease, including blood pressure, weight and cholesterol, to calculate a combined risk factor score.
Over one weekend and two week days the children were asked to wear a monitor that measured how physically active they were.
The researchers found that their risk score for cardiovascular disease decreased with increasing physical activity.
The lowest risk scores were found in the nine year olds who did 116 minutes of moderate to vigorous intensity activity and the 15 year olds who did around 88 minutes daily.
This would correspond to walking at a speed of around 4 km/h for 90 minutes.
Professor Lars Bo Anderson, from the Norwegian School of Sports Sciences in Oslo, and his team stress that the 90 minutes of daily exercise they are recommending for children would not have to be done in one chunk; it would be spaced over the day.
Little and often
For example, a child could walk or cycle to and from school, run around at lunchtime and play sports in the evenings and at weekends.
Neville Rigby of the International Obesity Task Force said children were being stifled from doing exercise.
"When you drive your child to the school gate in your Chelsea tractor you are not helping your child.
"Most kids in a previous generation had to walk to school, cycle to school or catch a bus."
Professor Chris Riddoch, head of the London Sports Institute at Middlesex University and one of the researchers who conducted the latest study, agreed, saying: "We have engineered a society that does not exercise - kids as well as adults."
He said children needed to be allowed and encouraged to be active at every opportunity.
"Every little bit helps. If we are not successful then the next generation of adults will be less healthy than we are and we are no role model."
He said much was being done to improve the situation but that unless things changed the NHS would crumble under the strain of treating escalating ill health.
A spokeswoman from the Department of Health said policy makers would consider the implications of the new findings "very carefully in the context of our efforts to halt the rise in obesity among children under 11 by 2010."
"It is important that we keep our recommendations under review as evidence like this comes to light," she added.
She said there were a number of schemes working to increase physical activity among young people, including issuing schoolchildren with pedometers - devices that measure how many steps someone takes.
The government also wants all school pupils to receive two hours of PE and sport a day by 2010.
Steve Shaffelburg of the British Heart Foundation said: "For children to develop a lifelong healthy attitude to physical activity, it will take a concerted effort from many groups working together to find long-lasting solutions."