Professor Charles Tyler, of the University of Exeter, is to present his findings in a key-note lecture at a symposium this week. He will explain that the offspring of such "transgender" or "intersex" fish can also be more sensitive to the effects of these chemicals in subsequent exposures.

Professor Tyler said: "We are showing that some of these chemicals can have much wider health effects on fish that we expected.

"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."

Tests showed 20 per cent of male freshwater fish, such as roach, 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, his study found.