Sewage water was pumped into the River Thames after Mogden treatment works struggled to cope with heavy rainfall.
Thames Water discharged storm water into the river just after midnight on Thursday after the works became “overwhelmed” following this week's wet weather.
A warning email was sent to rowers and other water users to alert them that partly treated storm water was to be discharged into the river.
It read: “Within the next hour, Mogden Sewage Treatment Works will be discharging heavily diluted storm water into the River Thames.
“We regret the necessity to discharge partly treated effluent in this way. Improvements are already under way at Mogden to reduce the chance of this happening in the future."
Thames Water faced criticism last year after it emerged 12.5 cubic metres of untreated sewage had been discharged into the river in November.
The revelation came just months after the water company admitted 20,000 tonnes of sewage had overflowed into the Thames on July 3, 2009, killing 200 fish.
This was followed by a further 900,000 tonnes just four days later.
Richmond Park’s former MP Susan Kramer met Thames Water officials following the incident, which she described as “depressing”.
Speaking about this week’s sewage overflow Thames Water said it hoped its multi-million scheme to improve its Mogden site would prevent future occurences.
A spokeswoman said: “Although regrettable, sewage discharges at Mogden are legally consented and occur when the site is overwhelmed following heavy rainfall.
“The system is designed to work this way to prevent excess flows backing up into people’s homes.
“This year we started a £675m scheme to modernise our five major sewage treatment works on the tidal River Thames, to significantly reduce sewage overflows and improve river water quality.
"The upgrade will enable Mogden to treat 50 per cent more sewage than at present.”
Work is due to be completed in 2013.