Most of the children [studied] are at high risk for type II diabetes

Associate Professor Sonia Caprio, study author

This is where cells gradually become less receptive to the effects of the hormone insulin, which reduces blood glucose levels after a meal or snack.

People with insulin resistance can go on to develop diabetes, in which blood sugar levels often cannot be brought under control without treatment.

The researchers found that even in the 11 to 18 age group, 21% had signs of insulin resistance.

Already diabetic

Four of the adolescents had "silent" diabetes, meaning they already had the disease, but had no obvious symptoms.

Associate Professor of Endocrinology Sonia Caprio, who led the study, said: "Most of the children are at high risk for type II diabetes.

"And, if they develop diabetes before the age of 20, they face a lifetime of being at very high risk for complications."

These complications include early heart disease, eye and kidney disorders.

Poor circulation in the legs and feet can also contribute to slow-healing ulcers, which can lead to permanent damage.

Lifestyle change

Currently, this kind of diabetes is often termed "late onset diabetes", as most new cases arise among people in middle age or older.

However, Dr Susan Jebb, from the Medical Research Council Centre for Human Nutrition in Cambridge, said that this was further evidence that the disease was likely to strike younger patients in future.

She said: "If you are 15 or 20 when diabetes strikes, you are much more likely to suffer from complications."

However, she said that there was no reason to suppose that the damaging changes already happening in pre-teen children could not be reversed if they began to lose weight.

She said: "Being overweight is a serious risk to your child's health - if you want to protect your children's, you need to change your whole family's lifestyle."

The study was published in the New England Journal of Medicine.