The charity said 738 people each day are told that they have type 2 diabetes, which is linked to being overweight.
"This clearly shows the frightening scale of what is fast becoming a national health emergency," the charity's Chief Executive Barbara Young said, warning of a strain to health services and disastrous consequences for patients.
"As the number of people with diabetes grows, we are likely to see even more people endure devastating health complications such as amputation and kidney failure and more people die tragically young."
Health authorities advise healthy eating and exercise to prevent developing the condition. Over a third of the British population have borderline diabetes and nearly four million people in Britain suffer from the condition, the charity said.
Almost a quarter of those aged over 15 in Britain were obese in 2012, according to the Organisation for Economic Co-operation and Development, compared to an average of 18.4 percent across the group of 34 wealthy countries.