More than one in 20 people who completed a questionnaire on Type 2 diabetes risk already had the condition, it has emerged.
A survey of 1000 people who filled in a Diabetes UK assessment found 6% said they went on to be diagnosed by a GP after being identified as at high risk of the condition.
The charity said the findings showed the potentially "life-changing" positive impact of undergoing a risk assessment, either through its online questionnaire or in pharmacy or GP surgery.
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Risk factors include being over 40 or over 25 if from a South Asian, African Caribbean, or Chinese background, having a close relative with diabetes, being overweight or having a large waist.
In the UK there are 3.2 million people living with Type 1 and Type 2 diabetes, a condition where there is too much glucose in the blood because the body cannot use it properly. A further 630,000 are estimated to have Type 2 diabetes but are undiagnosed.
It is thought that up to seven million people are at high risk of developing Type 2 diabetes - and if current trends continue, an estimate five million people will have the condition by 2025.
Barbara Young, chief executive of Diabetes UK, said: "If left unchecked, Type 2 diabetes can lead to devastating health complications such as amputation, blindness and stroke.
"This is why it is so important that people with Type 2 are diagnosed as quickly as possible.
"At the moment, too many people with the condition are not being identified quickly enough."