Diabetes UK estimates the number of people with some form of diabetes has passed three million for the first time. The figure rose by 132,000 in the past year alone.
A further 850,000 people have the disease but don't know it, the charity said.
As many as 24,000 people with diabetes die prematurely each year and Diabetes UK warned this could get worse without efforts to improve care.
Barbara Young, chief executive of Diabetes UK, said: 'We are hugely concerned that the number of people diagnosed with diabetes has reached three million for the first time and there is no reason to think this will mark the end of what has been a rapid rise in the condition.
'Instead, all the projections suggest that the three million figure will be a grim staging post on the road towards a public health emergency and this unfolding tragedy is already putting huge pressure on the NHS and will have potentially devastating consequences for those people who develop the condition.'
Seven million people may be at high risk of type 2 diabetes in the UK. Ms Young said more must be done to identify those at high risk so they can be helped to change their lifestyle.
Meanwhile, recent research has found eating too much sugar can directly increase the risk of diabetes. Before, scientists believed sugar intake played only an indirect role in development of the disease, by triggering obesity.