The agency which monitors environmental pollution said the sex changes were a result of the fish being exposed to treated sewage which still contained hormones produced naturally by women or as a result of them taking the contraceptive pill.

"There is sufficient evidence of harm to fish caused by sewage effluent that action needs to be taken now to find out how to control this," Andrew Skinner, director of environmental protection at the agency, said in a statement.

"We need to find out what is the best and most cost-effective way to remove these chemicals from sewage," he added.

It named the offending natural hormones as oestradiol and oestrone, and the synthetic culprit as ethinyloestradiol.

It said the findings had serious implication for future fish populations because anything more than a moderate change in a fish's sexual organs made it more difficult to reproduce.

The Environment Agency noted that while the phenomenon was being seen worldwide, Britain had the most comprehensive data on it.