One in 10 adults will have diabetes within a decade, latest estimates say.
The health timebomb is revealed in analysis predicting 5.5 million Brits will have the condition by the end of 2030.
Diagnoses of diabetes have doubled in the past 15 years.
The increase is thought to be due to longer lifespans and unhealthier lifestyles.
Nine in ten diabetes patients diagnosed have type 2, which is linked to obesity. Diabetes UK forecasted using trends from Public Health England plus data from Scotland, Wales and Northern Ireland.
Chris Askew OBE, the charity’s leader, said: “If we don’t act today, hundreds of thousands more will face the life-changing news they have type 2. We are at the tipping point of a public health emergency.”
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Some 850,000 Brits are thought to have undiagnosed type 2. The condition is estimated to cost £14billion a year.
It can lead to blindness, kidney failure, nerve damage and amputations. Diabetes UK is calling for check-ups and weight management schemes.
Mr Askew added: “With… care and support, diabetes complications can be avoided.”
Calorie-controlled diets from the NHS involving shakes and soups have been shown to put type 2 patients into remission.
Prof Jonathan Valabhji, of NHS England, said: “The NHS is already delivering the world’s largest type 2 diabetes prevention programme.”