Dr Jakko Tuomilehto, professor of public health at Helsinki University, said that his 10-year study showed that four hours' exercise a week achieved an 80 per cent reduction is the risk of developing diabetes. Volunteers who took other measures like eating a healthier diet and losing weight did not develop diabetes over the 10 years, he said.

Prof Tuomilehto stressed that doctors were not demanding too much of their patients. A man of average height weighing 197 pounds would be about 38 pounds overweight. A reduction of about 11lbs would have a beneficial effect in reducing diabetes risk.

He said that five per cent loss in body weight achieved a 40 per cent loss in internal body fat, another important risk for heart disease.

"It is important to remember that small things matter. If you eat a 50 calorie biscuit a day you will gain 5.5lbs in a year. But if you walk for a kilometer a day you will lose 5.5lbs a year. It takes 20 seconds to eat a biscuit and 20 minutes to walk it off. Half an hour's walking a day, in little bursts, 10 minutes at a time, is most effective for health."

Prof Tuomilehto said that diabetes was now common with a high lifetime risk. "By the time people have reached the age of 80, 40 per cent have developed Type II diabetes," he said.

Another 40 per cent have blood sugar abnormalities, known as glucose intolerance, which can lead on to diabetes. Type II diabetes can damage body organs, and is often "silent", not diagnosed.

Our study has shown that we can prevent diabetes and that means we can prevent heart disease by lifestyle measures alone.

"We had simple targets, a five per cent reduction in body weight, a reduction in dietary fats and an increase in activity of half an hour a day. Weight went down, glucose - blood sugar - levels went down and blood fats went down."

He said that when volunteers had achieved all the targets, including losing weight and reducing blood pressure, "not a single person developed diabetes".