Now parents are being warned that if they want their kids to grow up healthy they must take action before they reach 11, or it could be too late, according to research carried out in England.

The one-in-four figure was arrived at by Chinese University of Hong Kong researchers, while the English results came from a study of 5,000 obese children there.

"For many of these children, obesity will continue into their adult lives. Habits established early in life are always more difficult to change, so it is important to take action early," pediatric specialist Steve Lau Shing-chi said.

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"Obesity can be caused by genetics, disease and lifestyle. In addition, children tend to have inactive pursuits such as television viewing, surfing the internet and video games ."

According to the Department of Health, the percentage of obese children has increased to 21.3 in 2007/08 from 16.4 in 1997/98.

The Union Hospital in Sha Tin is organizing an eight-week weight- reduction course which involves dieting, exercise, drug treatment, counseling and family involvement.

"Family involvement is very important as they set examples," the hospital's nutrition and dietetics dietitian Joy Li Heung-ming said.

"Reducing weight is a step-by-step process and, with the cooperation of parents, children will be given replacements for high-calorie foods."

One of the first to join up for the course was eight-year-old Maximus Yu. His, Alison Yu, said: "He suffered from pain in his legs and excessive sweating. He is also experiencing emotional problems because of the derogatory nicknames given by his classmates."