Scientists from the University of the Saarland in Germany studied semen samples from 63 non-smokers and 53 heavy smokers.

They found men who smoke have lower levels of protamines. These are needed for correct chromosome segregation in sperm, and reduced levels can affect male fertility.

The authors said the results ‘suggest a negative biological effect of smoking on spermatozoa DNA integrity and protamine distribution’, and may be one explanation for the recent reduction in male fertility.

In a second study, researchers from the University of Copenhagen in Denmark found prenatal exposure to cigarette smoke could damage the fertility of the developing embryo.

They wrote: ‘Maternal cigarette smoking during the first trimester of pregnancy exerts a significant and dose-dependent reduction in the number of embryonic germ and somatic cells that may affect the future fertility potential of the exposed individuals.’

Both studies have been published online in Human Reproduction.