One in five adults in the UK is thought to be living with diabetes or prediabetes, which causes higher than normal blood sugars and carries a greater risk of developing the condition, according to new analysis.
The total includes millions of people who are not yet diagnosed, Diabetes UK said.
The charity warned that the country is facing a “hidden health crisis” and urged the Government to act in an open letter to the Prime Minister and Health Secretary.
It has called for those who are living with undiagnosed diabetes to be “urgently” identified, along with more investment for prevention programmes that promote lifestyle changes.
The new figures from Diabetes UK claim 4.6 million people in the UK have a diabetes diagnosis, the highest on record and an increase on the 4.4 million reported a year ago.
This includes about 8% with type 1 diabetes, which happens when a person cannot produce insulin, a hormone that helps the body turn glucose into energy.
Some 90% have type 2 diabetes, which happens when the body does not use insulin properly, while 2% have different and rarer forms of the condition.
Meanwhile, Diabetes UK estimates a further 1.3 million people have undiagnosed type 2 diabetes, with about 6.3 million people living with non-diabetic hyperglycaemia, also known as prediabetes.
If left untreated, prediabetes can develop into type 2 diabetes, although it can be reversed with lifestyle changes such as healthy eating and exercise.
The latest Diabetes UK figures mean more than 12 million people in the UK, or one in five adults, either have diabetes or prediabetes, the charity said.
Colette Marshall, chief executive of Diabetes UK, said: “These latest figures highlight the hidden health crisis we’re facing in the UK, and underline why the Government must act now.
“There must be better care for the millions of people living with all types of diabetes, to support them to live well and fend off the risk of developing devastating complications.”
In an open letter to Sir Keir Starmer and Health Secretary Wes Streeting, Diabetes UK called for the forthcoming 10 Year Health Plan to “prioritise urgently identifying those with pre-diabetes and those who have undiagnosed type 2 diabetes” through “improved NHS Health Checks”.
It also called for more investment in prevention schemes such as the NHS Diabetes Prevention Programme, which promotes lifestyle changes to those at risk of developing type 2.
The letter warns that diabetes “is a gateway to many other serious health issues”, like strokes, heart attacks and heart failure.
Last year, research by Diabetes UK warned the condition could cost the NHS almost £18 billion by 2035 if action is not taken.
Research published by the charity last May also highlighted a 40% increase in the number of people diagnosed with type 2 diabetes in the UK who are younger than 40.
Ms Marshall added: “With more people developing prediabetes and type 2 diabetes at a younger age, it’s also critical that much more is done to find the missing millions who either have type 2 diabetes or prediabetes but are completely unaware of it.
“The sooner we can find and get them the care they need, the more harm we can prevent.”
A Department of Health and Social Care spokesman said: “It’s extremely concerning that there are high numbers of adults living with type 2 diabetes. That’s exactly why preventative action is crucial.
“As part of our 10 Year Health Plan, we are committed to a shift from sickness to prevention to enable everyone to live a healthy life for longer.
“We are taking action to tackle type 2 diabetes-related lifestyle factors head on, including restricting junk food advertising on TV and online.
“We are also developing a new digital NHS Health Check that people can use at home to understand their risk of diabetes and how to reduce it.”