Italy is famed for its healthy Mediterranean diet, but alarming new figures show that it has a higher proportion of overweight children than anywhere in Europe.

While the rest of the world is encouraged to copy the traditional Italian menu by swapping junk food for fruit and vegetables, it seems Italians are forgetting the lessons they taught everyone else.

Traditional home-made meals and snacks are losing out to low-cost, calorie-packed fast food. Coupled with less physical activity, the results are evident with ever more "ciccioni" – fat children – on every street corner.

Dr Antonello del Vecchio, a practising doctor and spokesman for Slow Food, the international movement born 20 years ago, said: "Italians are eating less and less of the Italian diet and more and more fast food," he said. "For a long while, unlike northern Europe, we resisted, but now it's here and we're seeing the results."

More than a third – 36 per cent – of Italian children are either overweight or obese by the age of eight, according to a survey released this week by the Institute for Auxology of Milan, making Italy the worst in Europe in terms of obesity among young children.

"The seriousness of these figures must not be under-estimated," said Dr Giovanni Ancarani, president of the research institute. He noted that 10 per cent of the adult Italian population was officially obese and 35 per cent was overweight.

"That means 20 million Italians have weight problems," he said. Comparisons with past issues of the report suggest that the percentage of the Italian population with weight problems is growing by 0.5 per cent a year.

Another researcher, professor Michele Carruba, who also lectures at the University of Milan, blamed the couch-potato lifestyle. "Changes in diet and physical activity have brought an increase in obesity in adults and children in a relatively short time," he said.

Professor Carruba said that there were fears that the trend among youngsters meant the obesity epidemic in Italy "could explode". He said serious long-term consequences for those affected by obesity from a young age included an increased risk of diabetes, cardiovascular disease and cancer.