The form of diabetes more associated with adults is much higher among children than thought, figures suggest.
Type 2 diabetes is more associated with adults
Research in east London has revealed 22 children under 16 have type 2 diabetes, which is largely caused by obesity.
The findings suggest the latest audit of the type 2 diabetes in under 16s two years ago, which identified 100 cases in the UK, is a gross underestimate.
Experts said there may be up to 1,500 cases nationally now and warned the UK was sitting on a "time-bomb".
Ten years ago type 2 diabetes was unheard of among children, but it has begun to emerge as the obesity epidemic has exploded.
All but one of the cases identified in east London by Barts and the London and Newham University Hospital NHS trusts were from the ethnic minority population, which generally has a higher prevalence of type 2 diabetes.
Dr Jeremy Allgrove, a consultant in paediatric diabetes at the East London Centre for Paediatric and Adolescent Diabetes, said: "This is a time-bomb waiting to explode."
Diabetes UK chief executive Douglas Smallwood said: "These cases confirm a shocking trend that type 2 diabetes in children is increasing.
"Unless children are educated to eat a healthy diet and take up physical activity the number of children with the condition will continue to increase."
The charity, which has estimated there could be between 1,000 and 1,500 children with type 2 diabetes in the UK, has called for better access to specialist diabetes services and education.
People with type 2 diabetes, also known as non insulin dependent diabetes, do not make enough insulin or cannot make proper use of it.
Type 1 diabetes, which is more common among children, is not linked to obesity.
Professor Tony Barnett, head of the diabetes and obesity group at the University of Birmingham, said it was hard to estimate how many children have type 2 diabetes nationally, but said it could run into hundreds.
"Many of the cases we are seeing are among the Asian population. They seem to have a particular propensity to type 2 diabetes.
"It seems likely it is something to do with genes, a lack of exercise and diet.
"But the white population should not think they are not at risk.
"We have to address this. I would like to see food labelling being brought in and schools stop selling junk food."