Scientists have discovered a gene which may increase the risk of obesity by encouraging people to overeat.
Many children are seriously overweight
The researchers, from Imperial College London, say their work may lead to a test to identify vulnerable people and to take early preventative action.
The gene, GAD2, increases production of a chemical which boosts the appetite.
The study, published in Public Library of Science Biology, showed carriers were "significantly more likely" to give in to hunger.
Lead researcher Professor Philippe Froguel said obesity was a complex problem, which could not be entirely explained by one factor alone.
But he said GAD2 may be responsible for obesity in about one in ten seriously overweight people.
He said: "Having identified this gene, it may be possible to develop a screening programme to identify those who may be at risk of becoming obese later in life and take effective preventative measures."
One in five British men, and one in four women are obese. The rate of obesity has trebled in the last 25 years.
Doctors are particuarly concerned about the problem in the young. One in three children are overweight, and 17% are obese.
Research shows that being seriously overweight can take ten years off an average lifespan.
Obesity increases the risk of cancer, diabetes, heart attacks and strokes. It is thought to be responsible for 30,000 deaths a year in Britain and to cost the health service £2.5bn a year.
The Imperial study, of more than 1,200 people, identified two forms of the GAD2 gene. One protected against obesity, the other made it more likely by stimulating the appetite.
Thinner volunteers were found to be more likely to carry the protective form of the gene, while the other version was more common in obese people.
It seems to stimulate overeating by speeding up production of a chemical messenger in the brain called GABA, or gamma-amino butyric acid. When combined with another molecule GABA stimulates us to eat.
Dr Ian Campbell, chairman of the National Obesity Forum, told BBC News Online about 250 genes which may be linked to obesity were currently under investigation.
"Obesity is a very complex disease. If we can identify genes which have a particularly prominent influence on obesity that would be useful, but it wouldn't necessarily make treatment any easier."