Men's fast-shrinking powers to sow the seeds for future generations take two more body blows today.

Those considering "the snip" as a form of contraception are urged to "think very carefully" about the consequences if they ever wanted to reverse their vasectomies.

And those who smoke cannabis may also be damaging their fertility, according to researchers who found the drug's key ingredient impeded the motility of sperm and damaged its ability to penetrate women's eggs for fertilisation.

The double whammy comes in separate studies from the same university, Queen's in Belfast, to be presented to the British Fertility Society's meeting in Cheltenham.

One team found that sperm from 21 men who had vasectomies was about three times lower than that from men who had not undergone the operation.

They also found that couples undergoing fertility treatment where the male had had the procedure and reversal had failed were less than half as likely to get pregnant than couples with other fertility problems.