And analysis of official data reveals this includes more than 10,000 in Nottinghamshire.
Diabetes UK said about 850,000 people in the UK are thought to have type 2 diabetes, but have not had a formal diagnosis.
Separate analysis by the Dispatch of NHS and Government data reveals there are an estimated 10,300 undiagnosed diabetics in the NHS Nottingham and Nottinghamshire clinical commissioning group area, with most of these likely to have type 2.
Emma Elvin, senior clinical advisor at Diabetes UK, said undiagnosed diabetes risks serious complications.
She said: “Early diagnosis is the best way to avoid the devastating complications of type 2 diabetes, and offers the best chance of living a long and healthy life with the condition.
“Type 2 diabetes can sometimes go undetected for up to 10 years.
“While the symptoms can sometimes be tricky to spot in the early stages, it’s important to know the signs to look out for, and if you notice anything unusual, speak to your GP.
“We urge anyone concerned about type 2 diabetes to use Diabetes UK’s free online Know Your Risk tool – see riskscore.diabetes.org.uk/start
“It could be the vital first step towards getting a diagnosis and getting the right care to stay healthy.”
Diabetes leads to almost 9,600 leg, toe or foot amputations every year across the UK, according to the charity.
It is also one of the country’s leading causes of preventable sight loss – more than 1,700 people have their sight seriously affected by their diabetes every year.
Type 2 diabetes is far more common than type 1, and accounts for around 90 per cent of all adult cases in the UK.
The lifelong condition is caused by problems with the production of insulin in the body and is often linked to being overweight or inactive, or having a family history of type 2 diabetes.
It causes the level of glucose in the blood to become too high and can lead to a variety of serious health conditions, such as heart disease or a stroke.
The longer people with type 2 diabetes are undiagnosed, the higher their risk of serious complications.
Many people have the condition without realising as symptoms do not necessarily make you feel unwell, making the disease difficult to spot.
However, there are a few telltale signs to look for that could be a warning sign of type 2 diabetes.
What are the symptoms of type 2 diabetes?
Two common symptoms of high blood sugar – also known as hyperglycaemia – can be evident in the mouth, including a dry mouth and a breath that smells ‘fruity’, according to the NHS.
Other symptoms that could be a sign of type 2 diabetes include:
Peeing more than usual, particularly at night; Feeling thirsty all the time; Feeling very tired; Losing weight without trying to; Itching around your penis or vagina, or repeatedly getting thrush; Cuts or wounds taking longer to heal; Blurred vision.
Symptoms of hyperglycaemia in people with diabetes tend to develop slowly over a few days or weeks, and in some cases symptoms will not appear until blood sugar levels are very high.
Hyperglycaemia symptoms can also be caused by undiagnosed diabetes, so the NHS advises seeing a GP if this applies to you.
Who is most at risk of type 2 diabetes?
According to the NHS, you are more at risk of developing type 2 diabetes if you:
Are over 40 (or 25 for south Asian people); Have a close relative with diabetes, such as a parent, brother or sister; Are overweight or obese; Are of Asian, African-Caribbean or black African origin, even if you were born in the UK.
Type 2 Diabetes Prevention Week begins on Monday, May 23.
Research shows that for some people, combined lifestyle changes to their weight, diet and physical activity levels can cut the risk of type 2 diabetes by about 50 per cent.
How is type 2 diabetes treated?
Most people will need medicine to control their type 2 diabetes.
This will help to keep blood sugar levels as normal as possible to prevent further health problems and may need to be taken for the rest of your life.
A healthy diet and keeping active can also help to manage your blood sugar levels.
The NHS recommends eating a wide range of foods, including fruit, vegetables and starchy foods like pasta, and keeping sugar, fat and salt to a minimum.
Around 2.5 hours of physical activity is also advised per week.
There is evidence eating a low-calorie diet of 800 to 1,200 calories a day on a short-term basis – about 12 weeks – can help with symptoms of type 2 diabetes and some people have found their symptoms go into remission.
However, a low-calorie diet is not safe or suitable for everyone with type 2 diabetes, such as those who need to take insulin, so it is important to seek medical advice before going on this type of diet.