The figure includes about 4.3 million who have been diagnosed. An estimated 850,000 have not yet been formally diagnosed.
The research charity also estimates that more than 2.4 million people in the UK are at a high risk of developing Type 2 diabetes.
About 90 per cent of diabetes cases are Type 2, which is often linked to being overweight or inactive.
Diabetes UK expressed concern over the increasing number of people who are overweight or obese, which accounts for about 64 per cent of adults in England.
This trend is believed to contribute to the increase in Type 2 diabetes cases.
Type 2 diabetes is becoming more common among people under the age of 40, particularly in areas with higher levels of deprivation.
The risk factors for Type 2 diabetes are complex and include a person's age, family history, ethnicity and weight.
Diabetes UK called on the government to prioritise the illness in its coming major conditions strategy, aiming to prevent new cases and support those at high risk.
Without proper care and support, diabetes can lead to serious complications including the loss of sight, amputations, strokes and heart failure.
“Diabetes is serious and every diagnosis is life-changing,” said Chris Askew, chief executive of Diabetes UK.
“It’s a relentless condition, and the fear of serious complications is a lifelong reality for millions of people across the UK.
“These latest figures show we’re in the grip of a rapidly escalating diabetes crisis, with spiralling numbers of people now living with Type 2 diabetes and millions at high risk of developing the condition.
“But it doesn’t have to be this way. With the right care and support, cases of Type 2 diabetes can be prevented or put into remission.
"What we need to see is the will, grit and determination from government and local health leaders to halt this crisis in its tracks and improve the future health of our nation for generations to come.”
Diabetes UK urged the public to be aware of the symptoms of diabetes and to use the charity's free, online Know Your Risk tool to assess the likelihood they could develop the disease.