It provides the first firm evidence for something scientists have long suspected - that the huge quantities of synthetic hormones used over the past 25 years could be having an effect on life downstream.
The hormones are in the urine and faeces of women who take contraceptive pills. Similar but less potent hormone-like chemicals can also be formed when household and industrial detergents break down.
Scientists are uncertain whether one, or both of the two sources act on fish. Fish share some genetic elements with humans so can absorb the hormones.
The Department of the Environment funded the research after hermaphrodite roach were found next to a large sewage outfall a decade ago. For years the work was kept secret at the insistence of the now privatised water authorities, which met some of the costs.
Scientists at Brunel University in Uxbridge, west London, and at the Ministry of Agriculture's fisheries laboratories put caged carp and trout in rivers next to outfalls from 28 sewage works around Britain. Some or all of the fish died at several of these sites.
At 15, where the fish all survived, male fish and immature females began producing vitellogenin, a protein found in the yolk of fish eggs. It was roughly equivalent to a man beginning to lactate. The studies are continuing, and scientists recently found fish were being affected up to half a mile downstream of outfalls.
The scientists also exposed fish in the laboratory to different concentrations of a synthetic oestrogen hormone used in contraceptive pills, 17 alpha ethynyloestradiol, mixed into the tank water. They found that extraordinarily low concentrations of under one part in a million million affected fish in the same way as the sewage effluent. Even the most advanced chemical analysis techniques cannot detect the hormones at these levels.
The hormones are also likely to be finding their way into drinking water, but at much lower concentrations. Fish exposed to tap water showed no sexual changes.