Chemicals from contraceptive pills being flushed down household drains is causing male fish to turn transgender, a study claims.

About a fifth of male river fish are displaying feminine traits and even producing eggs, researchers said. Some have reduced sperm quality and display less aggressive and competitive behaviour, making them unlikely to breed successfully.

The chemicals causing these effects include ingredients in the contraceptive pill, by-products of cleaning agents, plastics and cosmetics. "We are showing that some of these chemicals can have much wider health effects on fish than we expected," said Charles Tyler, of the University of Exeter in the UK. "Using specially created transgenic fish that allow us to see responses to these chemicals in the bodies of fish in real time, for example, we have shown that oestrogens found in some plastics affect the valves in the heart," he added.

According to the study, 20 percent of male freshwater fish at 50 sites had feminine characteristics.

More than 200 chemicals from sewage plants have been identified with oestrogen-like effects and drugs such as antidepressants are also altering fish's natural behaviour, researchers said.

"Other research has shown that many other chemicals that are discharged through sewage treatment works can affect fish, including antidepressant drugs that reduce the natural shyness of some fish species, including the way they react to predators," Tyler said.