A report compiled for the Department of Health concludes that the data gathered significantly underestimates the problem because parents were allowed to opt out their children, and it was the parents of the fatter ones who chose to do so. Fewer than half the children who should have been weighed and measured were. In areas where the response rate was higher, more children were recorded as being obese.

The report, commissioned by the department from the Association of Public Health Observatories, says that 538,400 primary school children were weighed and measured — about 48 per cent of those eligible.

The results indicate that about 13 per cent of children aged 4 or 5 are overweight and 10 per cent obese, while among those aged 10 to 11, nearly 14 per cent are overweight and more than 17 per cent obese. But those figures are “likely systematically to underestimate the prevalence of overweight and obesity”, the report says.

Next year, if efforts to include more children are successful, more complete data will show an increase in obesity that may not be happening. So the database will be useless as an index of progress in conquering obesity, a target the Government is likely to miss.

• Obesity could be caused not only by what we eat but also by how it is digested. Researchers have discovered that the levels of two types of bacteria in the gut that help to break down foods are different in fat and thin people.

  The finding, reported in Nature, could lead to a better understanding of why some people may be prone to obesity, and help to find ways of preventing or treating it. The study was led by Jeffrey Gordon, of the school of medicine at Washington University in St Louis, Missouri.