Drinking a can of cola a day increases the risk of developing diabetes by a fifth, according to research.

The largest study of the link between soft-drink consumption and Type 2 diabetes in Europe has found that the sweetened beverages not only cause weight gain, which is associated with a higher rate of diabetes, but also increase the risk of the condition independently.

Almost one in 20 adults in the UK has diabetes, of which 2.6 million are diagnosed and 500,000 are undiagnosed. Rates are rising in this country and around the world, driven by Western lifestyles, and the number of cases is expected to exceed 4 million in the UK by 2025.

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Researchers from Imperial College, London, led the study of more than 12,000 people with Type 2 diabetes whose diets were compared with 16,000 controls in nine European countries, including the UK.

The results showed that people who drank one can of sweetened soft drink a day had a 22 per cent increased risk of diabetes.

The risk remained almost as high, at 18 per cent, even after account was taken of how overweight the individuals were and how much they ate.

Sweetened soft drinks contain a lot of calories which contribute to overweight and obesity, which in turn is a cause of diabetes. But the drinks appeared to increase the risk separately from this effect, possibly by triggering insulin resistance, reducing the body's ability to use glucose.

Diet drinks, with artificial sweeteners, did not appear to increase the risk once account was taken of individuals' weight and calorie intake.

Dr Dora Romaguera, of Imperial College, who led the study published in Diabetologia, said: "There was an association in normal weight individuals, overweight and the obese. Even in normal weight individuals, those who drank a glass of soft drink a day were more likely to develop diabetes."

No link with diabetes was found for those who drank fruit juice. But the researchers were unable to distinguish between pure unsweetened fruit juice and the sweetened and diluted fruit juices known as nectars, because the data was collected in the 1990s and no distinction was made.

Dr Romaguera said: "The hypothesis for fruit juice is different. We know they naturally contain sugar but fruits are not associated with an increase in diabetes, rather they are protective. It may be the anti-oxidants they contain that counter the effect of the sugar."