Presented at the European Congress on Obesity in Liverpool, the report by University of Essex researchers said that those who are too thin may face a greater threat to their health than those who are too fat.
The study of almost 10,000 children between nine and 16 in the East of England found that girls were significantly more likely to be underweight than boys, except among the black community, where boys were more likely to be underweight.
One in 17 white children was underweight for their age, according to the classifications used by the International Obesity Taskforce. Among Asians, the rate was one in 11.
Study authors Ayodele Ogunleye and Gavin Sandercock told the congress that malnutrition was a problem associated with the developing world but remained a major issue in the UK.
In their report they say: “The UK is obsessed with overweight and obesity – yet it is now accepted that underweight may contribute a much greater risk to health than overweight.
“Despite this, our only measurement programme at the National Obesity Observatory does not even mention underweight.”
LACA, the local authority school meal providers organisation, responded to the study by issuing a statement.
It said: “The study by the University of Essex, revealing that almost 6% (720,000) of Britain’s children are underweight, further highlights the importance of a healthy school meal.
“It also demonstrates the need for standards which ensure children and young people get the recommended nutrients and calorific intake at lunchtime.
“LACA not only believes that a healthy school lunch improves concentration levels in the classroom, but also plays a pivotal role in combating rising levels of both childhood obesity and malnutrition.”