Child obesity levels have been rising for decades
Scientists have discovered what they believe is a genetic cause of severe obesity in children.
The team concluded that the loss of a key segment of DNA can be to blame.
It said the findings might improve diagnosis of severe obesity - which on occasion has been wrongly attributed to abusive overfeeding.
The study, of 300 children with severe obesity by the University of Cambridge and the Wellcome Trust Sanger Institute, appears in Nature.
Some of the children in the study had been formally placed on the social services 'at risk' register on the assumption that their parents were deliberately overfeeding them. They have now been removed from the register.
Obesity is increasing throughout the world and is recognised as a major global public health concern.
Although much of the problem is due to lifestyle factors such as an unhealthy diet, and lack of exercise, some cases are thought to be down to genetics.
The latest study examined each child's entire genome, looking for deletions or duplications of DNA, known as copy number variants (CNVs).
Experts increasingly believe these CNVs play an important role in genetic disease.
By comparing the DNA profile of obese children with others of a normal weight they found certain parts of the genome were missing in the obese group.
In particular they zeroed in on a missing part of chromosome 16 which seemed to have a strong link to severe obesity.
Researcher Dr Sadaf Farooqi said: "Our results suggest that one particular gene on chromosome 16 called SH2B1 plays a key role in regulating weight and also in handling blood sugar levels.
"People with deletions involving this gene had a strong drive to eat and gained weight very easily.
"It adds to the growing weight of evidence that a wide range of genetic variants can produce a strong drive to eat.
"We hope that this will alter attitudes and practices amongst those with professional responsibility for the health and well-being of children."
Dr Matt Hurles, who also worked on the study, said: "This is the first evidence that copy number variants have been linked to a metabolic condition such as obesity.
"They are already known to cause other disorders such as autism and learning difficulties."
Dr Ian Campbell, medical director of the charity Weight Concern, stressed most children did not have significant genetic factors that predisposed them to obesity, and that lifestyle, diet and exercise remained important.
But he added that the causes of obesity - and the potential solutions - were complex.
He said: "The fact that several of the study children have been taken out of social care and returned to their parents as a result is disturbing in itself and must surely put an end to the claims by some that childhood obesity is a simple case of parental abuse.
"It clearly isn't. These families need our support."
Professor Alan Maryon-Davis, president of the UK Faculty of Public Health, said: "Far too many children are blamed or bullied because of their obesity, when it may be neither their nor their parents' fault.
"What's needed is a far better system of advice, help and support for overweight young people."