Researchers said that sedentary lifestyle was making children unfit. It was affecting children who were not obese also.

Essex University tested 600 10-year-olds who were not obese. The results showed a fall in fitness levels. It was concluded that a 10-year-old in 1998 could beat more than 90 per cent youngsters in 2008 in running.

A 20m shuttle run tests, also known as the bleep test, was conducted on 303 children from six schools in 1998. In 2008, the tests were repeated on a similar number of 10-year-olds according to the Archives of Disease in Childhood reported.

There was a considerable decline in the fitness level which is a cause of worry.

Lead researcher Dr Gavin Sandercock said, "The measurement of obesity may not be sufficient to keep an eye on children's future health.”

Professor Alan Maryon-Davis, president of the Faculty of Public Health, said, "We have been concerned about the sedentary lifestyles of children for some time. Worrying about obesity is right because it is linked to diabetes and heart disease."

Critics said billions of pounds to encourage children to remain active were not working.

Dr Sandercock added, "Children are not doing physical activities as before. They are using their spare time to play computer games and watching TV.”

Mike Penning, the shadow health minister, said, "Obesity was not the only indicator of children’s health.”

By 2011, children five to 16 years will be offered five hours of sport a week and 16 to 19 years will be offered three hours, the government informed.