The number of obese and overweight people in Britain has trebled in 14 years, research published yesterday shows.

A quarter of men and one fifth of women are now classified as obese, the study for the Food Standards Agency found. In addition, 41 per cent of men and 33 per cent of women are officially overweight. The figures come from the 2001 national diet and nutrition survey and show a startling rise in obesity rates since it was last conducted in 1987. Then, 8 per cent of women and 12 per cent of men were considered obese.

While older people are more likely to suffer from obesity, experts are concerned at the rising tide of weight problems among the younger generations. The 2001 survey shows that 18 per cent of men and 14 per cent of women aged 19 to 24 are obese.

Weight problems are classified by calculating a person's body mass index (BMI) - their weight in kilograms divided by their height in metres squared. A person with a BMI of 25 to 30 is overweight; more than 30 is classified as obese.

The survey also showed lack of awareness among people about their health and exercise levels. Participants were asked to keep a week-long diary of their physical activity. While 70 per cent of them claimed they were fairly or very physically active, only one-third of men and a quarter of women did the recommended levels of exercise.

More than half of all the adults in the study also had above-healthy levels of cholesterol in their blood.

Treating obesity and its effects costs the NHS more than £500m a year.