Two fizzy drinks a day could double the risk of diabetes - even if they are diet versions - a Swedish study has found.
Research by the Karolinska Institute on 2,800 adults found that those who consumed at least two 200ml servings of soft drinks daily were 2.4 times as likely to suffer from a form of type 2 diabetes.
Many fizzy drinks are sold in 330ml cans, meaning that one and a half cans would be enough to double the risk.
Those who drank a litre of such drinks saw a 10-fold rise in their chance of suffering from the condition.
The increased risks were the same regardless of whether the drinks were sugary or artificially sweetened, the research published in the European Journal of Endocrinology found.
Researchers said the sugary drinks may have induced insulin resistance, triggering the cases of diabetes.
The artificial sweeteners in the diet drinks may stimulate and distort appetite, they said, increasing food intake, and encouraging a sweet tooth. Such sweeteners might also affect microbes in the gut leading to glucose intolerance.
The research was a retrospective study, which relied on participants to recall their diet habits.
Josefin Edwall Löfvenborg, lead author, said soft drinks might influence glucose metabolism and insulin sensitivity, leading to the increased risk of latent auto-immune diabetes, a form of type 2 diabetes.
"In this study we were surprised by the increased risk in developing autoimmune diabetes by drinking soft drinks," he said. We next plan on investigating what could counter this risk."
More research was needed into the impact of diet drinks he said.
It was also possible that those consuming low calorie drinks may have switched to them after a long history of drinking sugary versions, which could explain the link with diabetes, he added.
Consumers of soft drinks were likely to have a lifestyle which was less healthy overall, a factor which the study tried to adjust for.
Tam Fry, from the National Obesity Forum, said: "This is yet another warning that sweetened drinks, though appearing harmless on the surface, can mess things up inside you. Why should you want to take that risk when a glass or two of water will slake your thirst and not put your health in jeopardy? "
Dr Elizabeth Robertson, director of research at Diabetes UK, said more research was needed into links between artificial sweeteners and diabetes.
Prof Christine Williams, Professor of Human Nutrition, University of Reading, said the findings were "very interesting" with both types of drinks appearing to have a large effect on diabetes risks.
"Even when the findings were adjusted to account for other factors that could explain the findings, such as greater energy intake, higher BMI or poor diet, the risks remained significantly higher for the higher intake groups," she said,
"A most interesting finding was that the higher risk was the same for both sweetened and artificially sweetened beverages, suggesting that greater risk of diabetes was not directly related to higher calorie intake, or adverse metabolic effects of sugar (in the form of sucrose) from the sweetened drinks."
Last year, a study by Harvard University suggested that two cans of fizzy pop could increase the risk of heart attacks and strokes.
The study found the drinks raised the risk of heart attacks by one third and the risk of strokes by one sixth.
Other studies have linked sugary drinks to a raised risk of prostate cancer.
A 15-year study found those drinking 300ml of fizzy drinks daily had a 40 per cent higher chance of the disease.
Earlier this week, a study found women who regularly consume soft drinks may be reducing their chances of getting pregnant.
The study of 524 patients found a link between artificial sweeteners, such as those used in "diet" sodas, and lower fertility rates, while use of sugar in soft drinks and added to coffee was associated with poorer quality of eggs and embryos.
One of Britain's leading fertility experts described the findings as "highly significant", and warned women not to underestimate the effects of food additives on their likelihood of conception.