A brisk 20-minute walk each day is all it takes to avoid dying prematurely, the findings suggest.

Scientists looked at the effects of obesity and exercise on 334,161 European men and women, whose progress was followed for 12 years.

They found that people who engaged in moderate levels of daily exercise - equivalent to taking an energetic 20-minute walk - were 16 to 30 per cent less likely to die than those classified as inactive.

Although the impact of exercise was greatest among normal-weight individuals, even those with high Body Mass Index levels saw a benefit.

Overall, avoiding inactivity theoretically reduced the risk of death from any cause by 7.35 per cent, said the scientists.

Having a BMI lower than obesity levels, defined as a score of 30 or more, was estimated to lower mortality by 3.66 per cent.

Keeping waists trim, irrespective of BMI, had a similar impact on death rates as exercise.

BMI is calculated by dividing a person's weight in kilograms by their height in metres squared and is a standard tool used to assess whether someone is overweight or obese.

At the population level, lack of exercise was thought to have caused almost 700,000 deaths across Europe in 2008.

Study leader Professor Ulf Ekelund, from the Medical Research Council Epidemiology Unit at Cambridge University, said the simple message was that just a small amount of physical activity each day could have substantial health benefits for people who were physically inactive, but that we should try to do more than just 20 minutes.

The findings are published in the American Journal of Clinical Nutrition. PA