Men who begin smoking as children may be more likely to go on to have overweight sons, a new study suggests.

According to the findings, men who started smoking before the age of 11 had sons who weighed up to 10kg (22lbs) more than their peers by the time they reached adolescence.

The UK study involved almost 10,000 fathers, half of whom had been smokers at some stage. Among these, 3% had started smoking regularly before the age of 11.

The men's sons were measured at the ages of 13, 15 and 17.

The researchers found that the sons of men who had started smoking before the age of 11, had the highest body mass index (BMI) every time they were measured, compared to the sons of non-smokers or men who began smoking later.

While a similar effect was seen in the daughters of early smokers, it was not to the same degree.

The researchers took into account other factors, such as the father's weight and genes, but none of these could explain the findings.

They suggested that the period before puberty may be a very sensitive time to be exposed to smoke and could lead to metabolic changes in future generations.

"This discovery of trans-generational effects has big implications for research into the current rise in obesity and the evaluation of preventative measures. It is no longer acceptable to just study lifestyle factors in one generation.

"We are probably missing a trick with respect to understanding several common diseases of public health concern by ignoring the possible effects of previous generations," said the study's senior author, Prof Marcus Pembrey, of the University of Bristol.

Details of these findings are published in the European Journal of Human Genetics.