Professor Sung and other researchers conducted the study in the 2005-6 academic year, measuring the weight, height, and waists of 14,842 students aged six to 18 at 36 schools.

A child was defined as overweight if the ratio of their height to weight was 20 per cent above the median.

'Obesity has been going up for some time now,' said Terry Ting Ho-yan, president of the Hong Kong Nutrition Association.

He said Hongkongers were not very knowledgeable about nutrition, and as they grew wealthier bought more food for their children. Adding to the problem, youngsters do not exercise as much as they did in the 1970s and 1980s, and have convenient forms of transport to save them the effort of walking, he said.

Official figures tell a similar story. The proportion of overweight children has risen from 17.8 per cent to 18.9 over the past three academic years, according to figures from the Department of Health's student health service. In 1995-6, the figure was 16.1 per cent.

Overweight children are more likely to have - or develop in later life - obstructive sleep apnoea, fatty liver, and cardiovascular diseases, said Albert Li Man-chim, a professor in the department of paediatrics who contributed to the study. They have a two times greater chance of having diabetes and are 10 times more likely to have sleep problems, he said.

To give educators an easier way of finding out whether children are obese, the researchers devised a tape that measures obesity by waist circumference. If a child's waist circumference is thicker than 95 per cent of the circumferences recorded for their age group in the study, they are considered obese. The university produced 5,000 tapes with funding and will distribute them to schools.

The researchers did not reveal a percentage for obese children in Hong Kong.

They chose that measurement because it was good for indicating health risk factors, and a tape was more convenient than using body mass indexes or going to public health centres for assessments, Professor Sung said.

The researchers found a high correlation between cardiovascular risk factors - including insulin resistance and hypertension - and waist circumference. When a child's waist is thicker than 70 to 80 per cent of their peers in their age group, their cardiovascular risk factors significantly increase. By placing the obesity cut-off at 95 per cent, the researchers provided parents with a sure indicator of obesity.

The tape could be used for mainland children, too, Professor Sung said, but not children of other ethnicities because they have different body builds.

Mr Ting said children should get active outdoors instead of sitting in front of a computer all the time, and eat more vegetables and less meat. Parents should accompany them to set an example.

The Department of Health's student health service centres measure every child who attends and provides dietary and health information. They monitor overweight children and refer high-risk children to paediatricians at the Hospital Authority.