A study has found that over the past 30 years, average calorific intake has increased by at least 500 calories a day, while exercise levels have changed little.

The study is the first major piece of research to quantify the relative contributions of food and exercise habits to the obesity epidemic.

Comparing data from the US from the 1970s and the early 2000s, it found that the diet of children contained 350 more calories, while adult diets rose by 500 calories.

If exercise levels had not changed at all, the weight of the average adult would have increased by an average of 24 pounds. In fact, the true figure was four pounds lower, suggesting exercise levels must have actually increased slightly over the period.

The researchers from the World Health Organisation Collaborating Centre for Obesity Prevention, based in Australia, said their findings - based on US data - were likely to be replicated across the Western world.

Last year the largest British study into obesity, backed by the Government and compiled by 250 experts, concluded that excess weight had become the norm.

It predicted that by 2050, 90 per cent of today's children will be overweight or obese – costing taxpayers an estimated £50 billion.

Professor Boyd Swinburn, from Deakin University, Victoria, in Australia, which published the study, said the findings would be similar for other developed countries.

He said: "This is a call to focus public health attention more on the energy intake side," he said.

"There is no evidence that a marked reduction in physical activity has been a contributor to this epidemic."

The study found the average US child is 9lb heavier than a those born in the 1970s, while adults weighed 19lb more.

For the US population to return to 1970s levels children would have to cut their intake by about 350 calories a day – equal to a can of fizzy drink and a small portion of fries, and adults by about 500 calories – about the same as a burger. Alternatively children would have to walk an extra two and a half hours a day, and adults nearly two hours.

"Getting everybody to walk an extra two hours a day is not really a feasible option for countering the epidemic," Professor Swinburn said.

"We need to limit our expectations of what an increase in physical activity can achieve."

Health professionals said the findings, presented on Friday at the European Congress on Obesity in Amsterdam, showed the need to tackle the availability and overpromotion of highly calorific foods.

The study follows research presented the day before which found that children who did more exercise at school did less when they got home.