The Lib Dems are calling for tunnel in Thames to not be delayed further
About one billion litres of raw sewage was dumped into the Thames during the storms in June, Liberal Democrats on the London Assembly have claimed.
Figures show between 24 and 29 June, the untreated sewage was pumped into the river due to the inability of the sewers to cope.
Last year Thames Water said a tunnel in the river may solve the problem.
But the Department for Environment, Food and Rural Affairs (Defra) said the tunnel could take 15 years to build.
The interceptor tunnel would help divert excess rainwater, which is causing ageing sewer tunnels to overflow.
A Defra spokesperson said: "The government has been active in seeking solutions to the problem of untreated sewage discharges to the tidal Thames.
"Further investigation has been requested by the Thames Tideway Strategic Study group regarding the proposed interceptor tunnel - which would take about 15 years to implement."
The statement said possible smaller scale measures which could bring earlier improvements to the problem of discharges were also being examined.
A spokesperson for Thames Water said it was aware of the problem and was currently working on interim solutions.
"Thames Water is spending £300m on major sewage treatment works. This will improve the situation but will not resolve it," he said.
"We are waiting on the government to take the next step."
The delay in building the interceptor tunnel has already increased estimated costs from £600m to £1.5bn.
Mike Tuffrey, London Assembly Liberal Democrat spokesman, said: "There must be no more discussion, delay or dithering on building this interceptor tunnel.
"The longer ministers drag their feet in making the inevitable decision to build the tunnel, the more this will ultimately cost the taxpayer to build."