About 20 million tonnes of sewage will flow into the River Thames every year unless a solution is found, the London Assembly has warned.
Thousands of fish died after storms forced sewage into the Thames
Thames Water was asked by the Assembly's Health Committee why it pumped one million tonnes of sewage into the river during August's storms.
The firm said the solution is a £1.5bn tunnel under the Thames but added it would take 10 years to build.
The Environment Agency said the health of the river's users could be affected.
Last October, the London Assembly warned that the city's Victorian sewage system could not cope with large amounts of rainwater falling during a short period.
In its report, 'London's Water Supply', the committee revealed that the system relied on releasing untreated sewage into the Thames when the drainage system was overwhelmed with floodwater.
Peter Spillet from Thames Water said improvements to sewage treatment works would take up to eight years and would not prevent all sewage flows into the Thames.
And Jon Goddard, from the Environment Agency, told the committee that around 5,000 people a week use the Thames for recreation and their health could be at risk if they use the river during the three days after an incident.
The Health Protection Agency and the London Port Authority are carrying out a study to see if the pollution is affecting people.
Hundreds of thousands of fish have been also been killed in the discharges.
Joanne McCartney, chair of the committee, said: "We just can't carry on like this.
"London is a world-class city - it is unacceptable that huge amounts of raw sewage, which includes human waste and sanitary products is released into our river up to 60 times a year and has been for years.
"[Water regulator] Ofwat has indicated it won't give Thames Water the green light to construct the tunnel. These bodies have to come up with a solution now.
"How long do Londoners have to wait until this repeated pollution of the Thames stops? "