Cannabis – which is the most widely-used illegal drug in Britain – tends to leave users feeling chilled out and relaxed.

But now researchers have revealed that it has the same effect on sperm and regular weed smoking can cut counts of the cells by as much as a third.

Dr Victor Chow, of the University of British Columbia in Canada, said: ‘The weight of the ­evidence is that marijuana ­probably has a negative impact not only for sperm counts but sperm function.’