The issue of underweight school children is being missed because of an "obsession" with tackling obesity, a group of researchers has claimed.
An Essex University study, presented at the European Congress on Obesity and involving 10,000 children aged nine to 16, found one in 17 was too thin.
Researcher Dr Gavin Sandercock said weighing too little was more damaging to health than weighing too much.
He warned that society was focused almost exclusively on obesity.
The research team looked at nearly 10,000 children aged nine to 16 in the east of England.
The height, weight, age and gender of the pupils was used to work out how many were too thin.
They showed 6% of all children were underweight, but it was more common in girls (6.4%) than boys (5.5%).
There were also large differences between ethnic groups. Asian backgrounds had the highest prevalence of being underweight at 8.7%.
It can lead to a lack of energy, weakened immune systems and delayed periods.
The problem of underweight children "may be more prevalent than we thought in the UK", said the scientists.
They said the fear of becoming obese, rising food prices, poor diets and a lack of muscle from low levels of exercise may all be playing a role.
"The fact is the UK is obsessed with overweight and obesity - yet it is now accepted that underweight may pose a much greater risk to health."
Dr Sandercock said attention had "absolutely" swung too far towards tackling obesity and warned children who were underweight could be being "missed".
He called for better training for GPs to spot the problem and new ways of helping parents.
Research published earlier this year showed that doctors may be missing the problem. University College London academics interviewed paediatricians at 177 hospitals in England and Wales and found a lack of knowledge about the warning signs of children being underweight.
Dr Hilary Cass, the president of the Royal College of Paediatrics and Child Health, said: "Dietary related problems in children are not uncommon, and it's been well documented that childhood obesity is prevalent amongst the UK population.
"Of course we also have to take seriously the fact that there are some children who are under-nourished or struggle with eating disorders."
The Royal College has developed growth charts for children between two and 18 which helps doctors tell if a child has a problem.
Dr Cass said: "Where children are severely underweight, it's often due to an underlying illness for which they'll need specialist medical help.
"But for the majority of cases, if we can get our children eating, choosing and ultimately cooking nutritious food, then we have a much better chance of preventing all sorts of dietary related problems - whether that's being over or underweight."