Scientists say they have destroyed the myth that you can be 'fat but fit' with research showing that obese regular exercisers are likely to die before slim, unfit people.
Obese people with high levels of aerobic fitness were 30pc more likely to die prematurely, compared with those who are slim but took little exercise, a study of 1.3 million men found.
The researchers tracked men for 30 years, before coming to the conclusion that being the right weight is the most important factor for long-term health.
Scientists said the findings demolished the myth that being fit could compensate for obesity.
A number of studies have suggested that obese people who were regular exercisers were at no greater risk of a potentially fatal illness than people of normal weight.
The new research, the largest study of its kind, was based on 18-year-old Swedish military conscripts whose aerobic fitness was tested by asking them to cycle until they had to stop due to fatigue.
They were then followed by the health researchers into middle age, for an average of 29 years.
Prof Peter Nordstrom, of Umea University, Sweden, said: "Unfit normal-weight individuals had a 30pc lower risk of death from any cause than did fit obese individuals."
He said the findings challenged the idea that obese people could compensate for their mortality risk by taking plenty of exercise.
Prof Nordstrom said: "These results suggest low BMI [body mass index] early in life is more important than high physical fitness, with regard to reducing the risk of early death."
Overall, men in the highest fifth of aerobic fitness had a 48pc lower risk of death from any cause compared with those in the lowest fifth, the study just published in the International Journal of Epidemiology found.
Such men had an 80pc lower chance of death associated with alcohol or drug abuse and a 59pc lower chance of suicide, and showed a 45pc drop in heart disease deaths.
But when such men were obese, they were still much more likely to die early compared with slim men.
Around two thirds of British people are overweight or obese, studies have shown. Earlier this month, England's chief medical officer suggested that obesity poses such a threat it should be treated as a "national risk" - alongside terrorism.
Professor Dame Sally Davies said the increasing problem of obesity was becoming so deadly that it was now threatening to overwhelm the NHS and cripple society's productivity.