Children living in inner-city areas are not getting enough exercise, the British Heart Foundation says.
Less than half of children said they walked to school (Picture: SPL)
A study funded by the charity found more than half of schoolchildren asked watched TV or played computer games for more than four hours every day.
Less than half said they exercised at break or walked to school, with south Asian pupils less likely to do so.
Researchers spoke to 3,500 pupils from white and south Asian backgrounds at five secondary schools in Leicester.
The foundation's study said the level of inactivity was found to be worse among girls, with only 12% saying they did any kind of physical activity at break time.
A history of type 2 diabetes or heart disease in parents is a risk factor for development of the conditions in their children.
But the researchers found that children with a family history of either condition were just as likely to be inactive as those without.
The British Heart Foundation said the findings were especially worrying because south Asians had a 50% higher death rate from heart disease than the rest of the population.
Professor Kamlesh Khunti from the University of Leicester, who helped research the study, said tackling obesity in children was crucial for the government and those working in healthcare.
He added: "This study shows that overall the physical activity levels in inner-city schoolchildren are very low.
"Parents, schools and community health providers need to address the results of these findings to reduce the future risks of developing diabetes and heart disease in children."
The British Heart Foundation described the findings as "alarming" but said the key to getting people of all ethnic groups to take the issue of obesity seriously was to increase awareness of the risks of heart disease.
Dr Mike Knapton, the charity's director of prevention and care, added: "Research like this will help us to better tailor health care and health education to today's multicultural UK population."
In January an MPs' report warned the government it had been too slow to react to rising levels of childhood obesity.
Despite a target to halt the rise of obesity among under-11s by 2010, the Commons public accounts committee said little concrete action had been taken.
But the government insisted progress was being made in encouraging healthy food choices and physical activity.