More than a third of overweight or obese teenagers don't see themselves as being too heavy and think their weight is about right, according to a Cancer Research UK study published in the International Journal of Obesity.

The study* used data from around 5,000 13-15-year-olds who were asked about their weight and if they thought they were too heavy, about right or too light.

Researchers then checked their answers against their Body Mass Index (BMI)** -- to see whether the reality matched the teenagers' perceptions of themselves.

Almost three-quarters (73 per cent) of the teenagers had a BMI within the normal-weight range. A fifth (20 per cent) had a BMI in the overweight category and seven per cent were categorised as obese. Of these, around 40 per cent thought they were about the right weight with very few (0.4 per cent) saying they were too light.***

Carrying excess weight increases the risk of up to 10 different types of cancer, including cancers of the breast and bowel. It is estimated that in the UK around 18,000 cases of cancer each year in people of all ages are related to excess weight.

The study also showed that more than eight in 10 of the normal-weight teenagers**** correctly identified themselves as being about the right weight. A small number of normal-weight adolescents (seven per cent) felt they were too heavy, and slightly more (10 per cent) thought they were too light, with girls more likely than boys to think of themselves as being too heavy.

Professor Jane Wardle, from the Cancer Research UK Health Behaviour Research Centre at UCL (University College London), said: "This study was a cause for celebration and concern. Young people who think they're overweight when they're not can sometimes develop devastating eating disorders, so we're delighted that most of the normal-weight teenagers had a realistic view of their body size.

"But we need to find effective ways of helping too-heavy teenagers slim down and maintain a healthier weight, and it's vitally important that we find out whether it helps if they are more aware of their weight status. There are no easy answers."

Julie Sharp, head of health information at Cancer Research UK, said: "Overweight teenagers are more likely to become overweight adults at higher risk of cancer. So it's important that young people who are too heavy have support to be more active and make healthy changes to their diet - being aware that they are above a healthy weight could be a first step. Making these changes as teenagers could help protect them from cancer as adults."