Data out on Wednesday are expected to show that sewage spills hit an all-time high over the last year, amid increased monitoring and wet weather.
The figures are likely to put fresh pressure on water companies and the Government to tackle the regular release of sewage, which is only permitted to happen in “exceptional circumstances” to stop it from backing up into people’s homes.
Last year’s data showed sewage was released into England’s waterways more than 300,000 times last year.
“We are in a tragic situation when elite athletes are issued with health guidance ahead of a historic race on the capital’s river,” James Wallace, the chief executive of River Action, said.
“For the safety of river users everywhere, rowers, communities and conservationists are uniting to ask the Government to enforce the law and to prosecute polluters.
The new guidance has been issued to rowing clubs across the country and included in briefing packs for Boat Race athletes.
“Taking action to improve the health of rivers is a key focus for us and we want to lead the way with our transparent approach to data,” A Thames Water spokesman said.
“We have experienced higher than average long-term rainfall across London and the Thames Valley with groundwater levels exceptionally high for the time of the year.
“The overflows are designed to operate automatically when the sewer network is about to be overwhelmed which then releases diluted wastewater into rivers, rather than letting it back up into people’s homes.
“We are working hard to make these discharges unnecessary and have published plans to upgrade over 250 of our sites, including a £100 million upgrade of our Mogden sewage treatment works in south-west London to treat the high volumes of incoming sewage and reduce the need for overflows during wet weather.”