London, July 10 (IANS): Over a third of overweight or obese teenagers do not see themselves as being too heavy and think their weight is just good, says a study.

"Young people who think they are overweight when they are not can sometimes develop devastating eating disorders, so we are delighted that most of the normal weight teenagers had a realistic view of their body size," said professor Jane Wardle from Cancer Research UK Health Behaviour Research Centre at the University College London.

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.

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

The study published in the International Journal of Obesity, found that almost three-quarters (73 percent) of the teenagers had a BMI within the normal weight range.

A fifth (20 percent) had a BMI in the overweight category and seven percent were categorised as obese.

Of these, around 40 percent thought they were about the right weight with very few (0.4 per cent) saying they were too light.

Eight in 10 of the normal weight teenagers correctly identified themselves as being about the right weight.

Carrying excess weight increases the risk of up to 10 different types of cancer, including cancers of the breast and bowel.

"It is 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," said Julie Sharp, head of health information at Cancer Research UK.