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About a million adults in England have type 2 diabetes but do not yet know it, putting their health at risk, experts warn.

The data, from the also suggests many more - perhaps five million - are on the cusp of developing it.

The condition causes high blood sugar levels that can lead to serious complications if left untreated.

Experts are particularly worried about the number of younger people at risk.

Older adults were more likely to have type 2 diabetes, but younger adults were more likely to be undiagnosed if they did have it - 50% of those aged 16 to 44 years with the condition were undiagnosed, compared with 27% of those aged 75 years and over, according to the estimates.

The ONS bases the calculations on data gathered from 26,751 adults taking part in an annual survey of the nation's health called the Health Survey for England. That data included blood tests looking at blood sugar levels.

Symptoms of type 2 diabetes can include:

People from an Asian, Black African or Black Caribbean ethnic background have a higher risk of getting diabetes.

Untreated high glucose levels can lead to long-term damage to blood vessels, nerves and organs. It is important that type 2 diabetes is spotted as early as possible.

Nikki Joule, from Diabetes UK, said: "Type 2 diabetes is a life-changing condition that often develops slowly, especially in the early stages when it can be very difficult to spot the symptoms.

"The figures published today by [the] ONS reveal a shockingly high number of people living with type 2 diabetes without a diagnosis, while millions more are at high risk of developing it.

"We're particularly concerned about the prevalence of pre-diabetes and undiagnosed type 2 diabetes in people from black and Asian backgrounds, and the worrying proportion of younger people who are undiagnosed, as we know type 2 diabetes is more aggressive in younger people.

"The findings are a reminder of just how important it is for type 2 diabetes to be detected and diagnosed as early as possible, so people can get treatment and support to reduce the risk of devastating complications and, importantly, be offered remission programmes where appropriate."