A third of male fish in British rivers are in the process of changing sex due to pollution in human sewage, research by the Environment Agency suggests.
Fish populations could be hit, the Environment Agency warns
A survey of 1,500 fish at 50 river sites found more than a third of males displayed female characteristics.
Hormones in the sewage, including those produced by the female contraceptive pill, are thought to be the main cause.
The agency says the problem could damage fish populations by reducing their ability to reproduce.
It said its study highlighted the need for water companies to develop new treatments.
There has been concern for some time that chemicals, known as endocrine disruptors, are causing fish to change sex.
The latest study is the first to show the scale of the problem in Britain.