Many families have refused to take part in an official report into childhood obesity, creating difficulties for researchers, it has emerged.
The Association of Public Health Observatories, which has undertaken the study on behalf of the Department of Health, said that less than half of the primary school children eligible to be weighed were measured.
Parents of many obese children were unhappy for their offspring to be weighed, the researchers believe.
The study found that around 13 per cent of children between the ages of four and five were overweight, while ten per cent were obese.
Nearly one in five ten and 11-year-olds were obese and the report claims that the figures are "likely systematically to underestimate the prevalence of overweight and obesity" as a result of families' refusal to participate.
The government is trying to combat the growing problem of obesity through a combination of healthy diet and lifestyle.
In addition, the National Institute for Health and Clinical Excellence (Nice) has recommended that obesity surgery be made available for exceptionally obese children on the NHS.