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Divide your weight in kilograms by your height in metres squared to find your body mass index (BMI). If the result is: Over 30 - you are medically obese 25 to 30 - you are overweight Under 25 - your weight is normal These calculations apply for adults only

The study, which began in 1947, recorded the heights and weights of participants at age 9, age 13 and then at age 50.

Their results revealed that although overweight teenagers were more likely to become fat adults, most fat adults were not overweight as young children.

Serious health problems

They also found that very thin children who became fat adults were more at risk of serious health problems.

Dr Ian Campbell, chair of the National Obesity Forum, said he was very interested in the results of the study.

He said: "It comes as no surprise that adolescents go on to become obese adults but these results must now call into question the demand for intervention into early obesity.

"We have been aware for some time that the rate of obesity in children is growing but until now we have always said fat children make fat adolescents make fat adults - this appears now not to be the case."

But Dr Campbell said despite these latest results there is still a big obesity problem in this country.

More than half the adult population of the UK is overweight and 20% of people are officially classified as obese.

One of the report authors, Charlotte Wright, who is senior lecturer in Community Child Health at Yorkhill Hospitals in Glasgow, said: "Our data suggests that whole population interventions in childhood directed at reducing body mass index in childhood may not benefit adult health.

"Meanwhile, underweight in childhood should still be a focus of concern since it offers no protection against adult obesity and is associated with increased risk of adult disease."

The research is published in the British Medical Journal.