A study of South Asian and white inner city schoolchildren has found "epidemic" levels of physical inactivity.
The survey funded by the British Heart Foundation (BHF) found most children do not take enough exercise.
It encompassed more than 3,500 pupils from five inner city secondary schools in Leicester.
Only half the children surveyed walked to school, with South Asian children particularly less likely to do so. And half of the pupils spent four hours or more a day watching television or videos or playing computer games.
Family history of 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 such a family history were just as likely to have sedentary behaviours as those without.
Professor Kamlesh Khunti, one of the University of Leicester academics behind the study, said: "People of South Asian origin comprise significant-sized minority ethnic populations in many countries worldwide.
"A consistent finding in South Asian migrant populations, wherever they are located, is a higher incidence and prevalence of premature coronary heart disease compared with the local population.
"Metabolic abnormalities precede the development of diabetes by some years and risk factors for cardiovascular disease in children often persist into adulthood. There is also evidence of increased risk of cardiovascular risk factors in children of South Asian origin compared to white children."
He said the need to put obesity prevention measures into place was a "major target" for the Government and healthcare professionals.
Prof Khunti added: "Inactive behaviour, such as watching television, may predict subsequent adult overweight and obesity in children and adolescent. This study shows that overall the physical activity levels in inner city school children are very low and parents, schools and community health providers need to address the results of these findings to reduce their future risks of developing diabetes and heart disease in children."
