Chemicals found in plastic wrappings and the environment could be behind the drop in sperm counts, scientists have suggested, after discovering that dogs are also losing their fertility because they live alongside humans.

Last year research showed that just 25 per cent of young men now produce good quality sperm and the average semen volume has declined by a quarter since the 1940s.

But scientists were unclear what was causing the problem, with everything from sunscreen to a rise in vegetarianism blamed for the decline.

Now researchers at the University of Nottingham have discovered that the same issue is also occurring in dogs whose sperm motility has fallen by around 35 per cent since 1988.

The study also showed that all dog food tested contained high levels of chemicals which are known to disrupt hormones.

Chemicals used to make plastics more bendy or furniture flame retardant can end up in food, both through leaching from wrappings, and because they are taken in by plants and livestock and so end up in the food chain. The same chemicals were also present in the dog testes.

Although the team say that it is too early to say that the chemicals are definitely causing infertility, they say that factories should review their processes to make sure food is as chemical free as possible.