Researchers found drinking pure fruit juice was not associated with diabetes risk.
Dr Mann, director of the Edgar National Centre for Diabetes Research, said the study was further evidence that sugary drinks were bad.
But he cautioned against taking the study's can-a-day finding too literally, saying it was difficult to disentangle the risk of diabetes from obesity.
"If we are too literal about our interpretation of these things ... there's a danger that the results are rubbished, when actually the real bottom line is that there is unquestionable evidence that sugary drinks are not good and we need to restrict them.
"And yes, even one sugary drink probably has some effect, but I'm a little bit nervous of making too much of the precise number, when actually there's confidence intervals attached to that number.''
The study's lead author, Dr Dora Romaguera of Imperial College London, said the increased risk of type 2 diabetes among soft drink consumers in Europe was similar to that in North America - where the risk was 25 per cent for each sugar-sweetened soft drink consumed a day.
"Given that people are drinking more and more sugary drinks in Europe, we need to give out clear messages about their harmful effects.''
The study was published in Diabetologia, the journal of the European Association for the Study of Diabetes.
