Men who smoke tobacco could be putting their children at greater risk of cancer because of damaged sperm, according to scientists.

The study, made at the University of Birmingham, is not about passive smoking, the effect of smoke on children's lungs. It is about the damage smoking can cause long before babies take their first breath.

Doctor Tom Sorahan and his colleagues have found new evidence that smoking can damage sperm.

The research, published in the British Journal of Cancer, suggests that a child fathered by a man who smokes more than 20 cigarettes a day has a 30% greater risk of getting cancer.

This figure takes into account a range of other factors, including age, social class and whether or not the child's mother smoked.

The scientists, who looked at more than 2,500 children over a period of five years, confirm earlier British and American evidence that tobacco smoke damages the DNA, the genetic material of human sperm.

The health message to potential fathers: In Europe alone there would be around 1,000 fewer cases of childhood cancer every year if fathers would not smoke.

Although smoking does not seem to have a directly comparable effect on the DNA of mothers, there is already proof from other studies that mothers who smoke give birth to lighter, sicklier babies.

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