There are nearly 7,000 children and young adults under 25 with type 2 diabetes in England and Wales – about 10 times the number reported before, according to Diabetes UK.
Its analysis of 2016-17 data found most were being treated in GP practices rather than in specialist units.
The condition is linked to obesity and is more aggressive in children.
Child health experts said the government should act now to implement proposals to cut childhood obesity.
The Obesity Health Alliance said it was "hugely concerning" to see so many young people diagnosed with type 2 diabetes.
"We know that obesity contributes to the development of this and other serious health conditions, and with one in three children leaving primary school with excess weight or obesity, these findings are worrying but sadly not surprising," said Caroline Cerny, from the Alliance.
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.
- 11 five to nine-year-olds
- 196 10 to 14-year-olds
- 1,246 15 to 19-year-olds
- 5,383 20 to 24 year-olds
But 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.
These units provide specialist support to young people.
Some may be at risk of the condition progressing quickly, creating other health complications, such as blindness, amputations, heart disease and kidney failure.
GPs also provide lifestyle advice and support and many young people are treated in primary care.
Family history and ethnic background, as well as obesity, are factors in children and young people developing type 2 diabetes.
What is type 2 diabetes?
- It is a common condition that causes the level of sugar (glucose) in the blood to become too high
- It's caused by problems with a chemical in the body (hormone) called insulin
- Type 2 diabetes can cause symptoms like excessive thirst, needing to pee a lot and tiredness
- It can also increase the risk of getting serious problems with the eyes, heart and nerves
- It's a lifelong condition which may mean a change in diet, medicine and regular check-ups
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 said children and young people with type 2 should have access to expert treatment from healthcare professionals trained to manage the condition.
Prof Russell Viner, president of the Royal College of Paediatrics and Child Health, said these new figures emphasised the need to act.
"For many children, the development of type two diabetes can be prevented with lifestyle changes, but this isn't easy – they need support.
"That's why we were pleased to see the ambitious proposals set out in chapter two of its Childhood Obesity Plan. We urge the government to maximise their impact by introducing them all and doing so quickly."
A spokesman for the Department of Health and Social Care said it was committed to halving child obesity by 2030.
"We will be launching consultations to restrict promotions in shops for sugary and fatty foods, as well as a 9pm watershed ban on advertising.
"The upcoming NHS long-term plan will have prevention at its core and build on our existing work to keep people healthy and well."