The number of children and young adults with type 2 diabetes is about 10 times higher than previously thought.
According to Diabetes UK, there are nearly 7,000 children and young adults under 25 with type 2 diabetes in England and Wales.
The condition is linked to obesity and is more aggressive in children.
The new figures come from the latest National Diabetes Audit for 2016-2017, which contains information on cases of type 2 from 95% of GP practices in England and Wales, as well as numbers treated in specialist paediatric units.
It found that a total of 6,836 children and young people aged under 25 were being treated for the condition.
A previous report, from the Royal College of Paediatrics and Child Health, suggested the figure was 715 – the number of children and young people receiving care for type 2 diabetes from paediatric diabetes units in England and Wales.
Bridget Turner, director of policy and campaigns at Diabetes UK, said type 2 diabetes can be devastating for children and young people.
“We need to encourage healthy living by providing clear and easy to understand nutritional information about the products we are all buying, and protect children from adverts for foods that are high in fat, salt and sugar,” she added.
A spokesman for the Department of Health and Social Care told BBC News it was committed to halving child obesity by 2030.
“The upcoming NHS long-term plan will have prevention at its core and build on our existing work to keep people healthy and well,” he added.