Male fish exposed to treated sewage effluent in a Kent river have been showing signs of "feminisation".
Oestrogenic chemicals entering the river are causing the problem
The Environment Agency has found that oestrogen enters the River Stour at Ashford when the nearby Bybrook sewage works discharges its end product.
Tests on male fish taken from the river show they have immature eggs that are normally only found in female fish.
Chairman Sir John Harman said the problem was widespread across the UK, and roach seemed to be more affected.
About 25% of male roach around the sewage treatment works have been found to have varying degrees of "feminisation".
Hormones in the sewage, including those produced by the female contraceptive pill, have been cited as the cause.
The fisheries technical team leader in Kent, John Witmore, said natural and artificial hormones were to blame.
"We call them oestrogenic chemicals because they mimic the effect of oestrogen which is basically a female hormone," he explained.
Research is being undertaken into how serious the problem is for fish populations, and whether or not it poses a risk to human health.
The Environment Agency is also looking into what can be done about it.
Barbara Young, chief executive, said: "At a time of growth in Ashford and the South East, it is important that development goes ahead in a way that will protect and enhance the natural environment.
"By carrying out this crucial research now, we can find ways of ensuring the future development of Ashford does not harm the valuable, but sensitive, River Stour."