Fat children are born, not made, a study of twins has found. Instead of blaming parents of overweight children, we should accept that factors which determine body size are largely outside their control, researchers say.

A survey of 5,000 pairs of twins found variation in the children's waist measurement and body mass index was 77 per cent attributable to genes and 23 per cent to environment. Researchers compared identical pairs of twins, who share the same genes, with non-identical twins who share only half of their genes.

Jane Wardle, director of Cancer Research UK's Health Behaviour Research Centre, said: "Contrary to the widespread assumption that family environment is the key factor in determining weight gain, we found this was not the case. The study shows it is wrong to place all the blame for a child's excessive weight gain on parents; it is more likely to be due to the child's genetic susceptibility."

The findings are reported in The American Journal of Clinical Nutrition.

One third of British children are overweight or obese, a figure projected to rise to two thirds by 2050 if nothing is done. Overweight children are more likely to become overweight adults with higher risk of diabetes, heart problems and cancer.