River Thames water quality needs to be monitored more closely to enable public warnings to be given when sewage flows into it, a survey has suggested.
The Thames is popular with rowers, canoeists and people fishing
The Health Protection Agency found that heavy rainfall causes untreated sewage to contaminate the river, increasing health risks for up to four days.
It found river water at Kew, Barnes and Putney, south-west London, did not meet standards for recreational use.
The City of London Corporation called for a pollution early warning system.
The Health Protection Agency (HPA) carried out water quality tests for 90 weeks in sections of the Thames popular with rowers, canoeists and people fishing at Kew, Barnes and Putney.
They found that the water contained bacteria and viruses which could cause gastroenteritis and other infections.
Sewage treatment works are unable to cope with excess surface water following heavy rainfall, leading untreated sewage to flow into the river.
This increased health risks to river users for up to four days afterwards - not two days afterwards as previously thought, the HPA found.
The survey was commissioned by the City Corporation, whose port health official Jon Averns described the findings as "very significant".
Mr Averns said the health risks must be reduced by implementing a system to warn river users of the extent of pollution.
Dr Susanne Surman-Lee, who led the HPA survey, said: "People using the Thames for recreation purposes should not be alarmed by these results.
"They should however be aware of the possibility of contracting infections as a result of using the water.
"They should therefore ensure they wash their hands thoroughly after using the river, particularly before eating or drinking, and try not to swallow any water if they should fall in."