The obesity epidemic will not be reversed by urging people to exercise more, because they have too little time to spare, researchers claim.

To make an impact on levels of obesity, severely overweight people would have to exercise for several hours a day, when they could find it easier to lose the weight by eating less, they said.

An obese person with a body mass index of 35 could reach a more healthy weight and BMI of 22 by reducing their calorie intake by one third. "That is equivalent to exercising for around five hours a day. That is not realistic," said Professor John Speakman of the energetics research group at Aberdeen University.

A study by Speakman's group found that contrary to popular belief, our lives have not become more sedentary in recent decades. Physical activity has remained the same for at least the past 25 years, while obesity rates have soared.

Since 1985, the typical man in Britain has burned off 1,380 calories a day through exercise, compared with 950 calories for women. The cause of rising obesity is linked to the population eating more high-calorie food than exercising less, Speakman said.

"Promoting exercise is a good idea, but if you want to tackle the obesity epidemic it is not the solution. Weight loss is not a key benefit from exercise," he said. Foregoing a small sandwich was as effective as a one-hour run, he added.

About a quarter of British adults and a fifth of children are obese and this figure is rising. An estimated 30,000 people die prematurely across the country from obesity-related illnesses each year.

"You cannot exercise your way out of the obesity epidemic. It would take an enormous intervention in physical exercise," Speakman said.

"It is important for policymakers to realise that if they want to promote weight loss in overweight and obese people, the most effective way is through healthy eating and diets."

However, the report says exercise protects against heart disease, type 2 diabetes, osteoporosis and high blood pressure.