An alarming increase in obesity among children is due to their increasingly lazy lifestyle, according to a study.

The number of officially obese six-year-olds has doubled in the last ten years while the proportion of obese 15-year-olds has trebled.

And while poor diet plays its part, inactivity is the chief culprit.

Slumped in front of the TV, hunched over a computer game or being ferried by their parents in the school run - youngsters are not burning off the calories they used to. As a result, one in six teenagers is officially classified as obese and likely to face decades of health problems.

Data from the Health Survey of England 1996, published in The Lancet, shows that children of all ages are more overweight than experts expected.

Dr John Reilly, senior lecturer in human nutrition at Glasgow University, who led the study, said the results were ‘alarming’.

He added yesterday: ‘The figures are much more dramatic than we expected compared with trends 10 years ago. It is very worrying because the problem is getting worse as children get older. Many are on a slippery slope to obesity from an early age and it will be difficult for them to get back to a healthy weight.’

A total of 2,630 children aged six to 15, were measured for height and weight. Compared with ten years ago when five per cent of six year olds were obese, the figure had risen to 10 per cent.

In the mid 1980s just five per cent of 15-year-olds were obese but the figure now is 17 per cent. Obesity has known health risks in later life, particularly an increased chance of suffering high blood pressure, diabetes and heart disease.

Being obese means the children are at least 20 per cent heavier than their recommended weight for height.

Doctors also used a measure called the body mass index to define obesity in the children. Body mass index is calculated as weight in kilograms divided by height in metres squared. A figure of 20 to 25 is regarded as healthy. Over 25 is overweight and if it is over 30 you are obese.

Dr Reilly said: ‘Children live in environments that encourage obesity and the problem gets worse as they grow older. Inactivity is the root cause and there is evidence to show children are doing less and less.’

Dr Reilly said it was ironic that so many teenage girls were dieting and worried about their weight. ‘Yet all teenagers, boys and girls, are getting fatter, ’ he said.

According to research, the ideal weight for a girl of six is around 2st 12lbs (18 kilograms) and for a boy 3st 6lbs (22 kilograms) with both measuring around 3ft 8ins (114 cms. The ideal weight for a boy and girl aged ten who are 4ft 4ins (134 cms) is around 5st (32 kilograms).

For a 15-year-old girl of 5ft 3ins (162 cms) the ideal weight is around 8st 4lbs (53 kilograms) and for a 15-year-old boy of 5ft 5ins (169 cms) around 8st 9lbs (55 kilograms).

Dr Reilly said the Government had to take the issue seriously and set targets to reduce the proportion of overweight youngsters.

He added: ‘We’ve never had the evidence before showing how alarming the problem is. At the very least we have to keep monitoring it but we also need more funding to investigate interventions that are effective.’

CASE HISTORY Weighing 18st and standing 6ft tall, 15-year-old Mark Cullen comes in very handy on the rugby pitch. But he knows he needs to embark on a healthy eating and exercise regime to get his weight down to achieve his ambition of joing the Royal Navy once he completes his A-levels. Mark, who plays rugby for his county side Sussex, believes he would be a more effective sportsman if he was in better shape.

His mother Geraldine is also aware of the possible medical problems inherent in being overweight. Mrs Cullen said: ‘My father died at 43 of coronary thrombosis and his brother at 45. I take tablets for High cholesterol so I worry about Mark.’

Mark, who lives with his mother and father, Richard, in Hailsham, East Sussex, attended Britain’s first ever fat camp in July. The youngster feels lucky because he has never been teased at school. He added: ‘I do feel self conscious about my weight when I take off my T-shirt. I know I’d be a better sportsman if I lost weight. I already eat very little. I don’t really know why I’m so heavy.’