More than 80% of the male bass fish in Washington's major river are now exhibiting female traits such as egg production because of a "toxic stew" of pollutants, scientists and campaigners reported yesterday.

Intersex fish probably result from drugs, such as the contraceptive pill, and other chemicals being flushed into the water and have been found right across the US.

The Potomac Conservancy, which focuses on Washington DC's river, called for new research to determine what was causing male smallmouth bass to carry immature eggs in their testes. "We have not been able to identify one particular chemical or one particular source," said Vicki Blazer, a fish biologist with the US geological survey. "We are still trying to get a handle on what chemicals are important."

But she said early evidence pointed to a mix of chemicals – commonly used at home as well as those used in large-scale farming operations – causing the deformities. The suspect chemicals mimic natural hormones and disrupt the endocrine system, with young fish being particularly susceptible.

The chemicals could include birth control pills and other drugs, toiletries especially those with fragrances, products such as tissues treated with antibacterial agents, or goods treated with flame retardants that find their way into waste water. However, Blazer also pointed to runoff from fertilisers and pesticides from agricultural areas.

About 5 million people live in the greater Washington area, and 90% of them get their drinking water from the Potomac.

There is evidence that the anomaly is not confined to the Potomac, one of the largest rivers on America's Atlantic coast.

A report last year by the US geological survey found intersex fish in a third of 111 sites tested around the country. Of the 16 fish species studied, the condition was most common in smallmouth and largemouth bass and among males, although researchers also discovered the occasional female fish with male characteristics. The researchers studied sites along some of America's greatest rivers from the Mississippi to the Rio Grande.

"We need to get these toxins out of our river water," said Hedrick Belin, the president of the Potomac Conservancy. The campaign group called for $3m (£1.9m) in research over three years on endocrine disrupters on their effects on fish. It also called on the authorities to involve pharmaceutical companies in the safe disposal of drugs, and to invest in technology that can filter out endocrine disrupting compounds.