Amanda Sexton doesn't know why her 10-year-old son Laurence started overeating, but food has been the focus of his life ever since he started school. Now 10 stone, he has had to endure the taunts of school bullies for as long as he can remember.

'It really became a problem when Laurence was six,' said his mother, who lives in Crystal Palace, south London. 'He would devour any junk food he could get his hands on and was prone to binge eating. But still he was always hungry.'

Laurence is one of thousands of children across Britain suffering from obesity, a condition which threatens to become the public health catastrophe of the decade. At the heart of the problem lies the fact that kids are becoming more and more slothful, increasingly eating junk food without doing the exercise which would help them burn off the calories.

The Association for the Study of Obesity conference in Bristol will hear this week from Dr Angie Page, who monitors activity rates among 70 overweight children. She has found that some are only doing 32 minutes of moderate physical activity a day.

Page, a lecturer at Bristol University told The Observer: 'We found some were doing less than one or two minutes of moderate activity an hour, which is a lower figure than we've seen from any other data.'

Around 30 per cent of the children she recorded had very low levels of activity. 'Teenage girls tended to do less than boys, and the older children did less than the younger ones. It's very hard for the parents; a lot of them feel quite helpless. Most of them know their children are fat, but they need support to help them change their lifestyles.'

Putting on the pounds pulled Laurence Sexton into a vicious circle: as he grew bigger, the taunts got worse. His mother said. 'He would come home in tears. He thought nobody wanted to be his friend because of his weight. I went to see the doctor, who referred me to a dietician, but whatever we tried seemed to have no effect.'

The turning point came when she saw a TV documentary about the Carnegie International Weight Loss camp in Leeds. Laurence went this summer and was taught to read food labels, eat healthily and do exercises. He lost a stone.

'It's a great place, there's nowhere else like it in this country,' said his mother. 'Just being around other kids who are the same really helps.'

Such programmes may be the answer but there are very few places, and it is hard to get families to admit that their child has a problem. Hospitals also have scant resources to deal with the rising number of patients with conditions associated with being very overweight.

At Bristol, Page's colleague, Dr Julian Shield, a senior lecturer in metabolics, has already diagnosed early onset Type II diabetes in eight young patients - a condition brought on by obesity.

'Children with a weight problem are all too aware society doesn't view them favourably,' said Shield. 'As they get fat, they become more reclusive, go out less, and do less exercise.' Many turn to 'comfort-eating' because of their unhappiness.

With around 10 per cent of children now diagnosed as overweight, and between 2 and 3 per cent as obese, doctors are struggling to see what can be done to shift an entire generation's eating habits.

Exercise is crucial. Fewer than 5 per cent of children walk or cycle to school, compared with 80 per cent 20 years ago. And many schools are blamed for adding to the problem by selling off playing fields, having no proper sports halls and doing too little to offer healthy lunches.

The food industry must also take blame for aggressively marketing food high in fat and sugar at children, according to Dr Susan Jebb, of the Medical Research Council's Human Nutrition Research Centre in Cambridge. One of her biggest concerns is fizzy drinks.

'We know around one-fifth of children get more than 20 per cent of their total energy from sugar, and around one-quarter of that 20 per cent comes from soft drinks,' she said.

Jebb is also worried about 'super-sizing', the making of king-size chocolate bars and bags of crisps. 'These are attractive to poorer families because you are getting more food for the same price,' Jebb said. "But if you put a big bag in your child's lunch box, do they save half for the next day? Of course not.'

Amanda Sexton would readily agree with the experts who spoke last week at an EU summit on obesity in Copenhagen. They want governments to begin to treat the fast-food industry as they do tobacco companies, forcing them to put health warnings on high-fat food. They have also called for a ban on vending machines in all schools.

The aggressive marketing of fast food is a modern evil, according to Amanda. 'There are countless adverts on television and in magazines, and on the street you see fast food chains on every corner. People go on about tobacco advertising, but I would ban food adverts if I could.'

Additional reporting by Indira Das-Gupta