Obesity is associated with a large number of very important diseases

Professor Peter Kopelman, Royal London Hospital

In the US, type two diabetes, which usually develops in obese people later in life, has been found in teenagers.

The World Health Organization has said obesity is the biggest unrecognised health problem in the world.

Professor Peter Kopelman runs the obesity clinic at the Royal London Hospital.

He told the BBC: "There is a major concern about the increasing proportion of the population who have a major weight problem.

"Obesity is associated with a large number of very important diseases. It is a major health risk."

Professor Kopelman said the problem was spreading to developing countries. As they become more affluent the proportion of obese people is increasing.

He said: "There is a sort of double whammy. There is a population who are under-nourished, and importantly an increasing proportion of the population who are becoming obese."

No exercise

Professor Kopelman said bad diet was part of the problem, but lack of physical exercise was also a significant contributory factor - particularly in the UK.

"The fact is that we are in danger of becoming a nation of couch potatoes," said Prof Kopelman.

He said only a small number of people were aware of the problems of obesity, partly because the impact on health tended to come many years later.

Obesity is associated with an increased risk of heart disease, high blood pressure, diabetes and respiratory problems.

Professor Kopelman said that if people did lose weight - even relatively modest amounts - their health would improve considerably.