Parents of obese children rarely spot that their child is overweight, doctors warn.
A study of 2,976 children found that only four parents thought their child was very overweight, despite doctors identifying 369 children as such.
The research, published in the British Journal of General Practice, showed that nearly a third (31%) of parents underestimated their child's weight.
Researchers said children were unlikely to lose weight and adopt a healthier lifestyle unless their parents could recognise the issue.
They said parents may be underestimating their child's weight out of fear of being judged and an unwillingness to label a child as overweight.
Changes in public perception of what is a normal weight is also affecting parents' views, the study added.
"If parents are unable to accurately classify their own child's weight, they may not be willing or motivated to enact the changes to the child's environment that promote healthy weight maintenance," said senior author Dr Sanjay Kinra, from the London School of Hygiene & Tropical Medicine.
Co-author Professor Russell Viner, academic paediatrician at the UCL institute of Child Health, said: "Measures that decrease the gap between parental perceptions of child weight status and obesity scales used by medical professionals may now be needed in order to help parents better understand the health risks associated with overweight and increase uptake of healthier lifestyles."