A planned “super sewer” would need to be twice as big to stop waste spilling into Thames, a water boss has said.

Sarah Bentley, the Thames Water chief executive, admitted to MPs that her company’s performance was “unacceptable” as it continues to dump millions of tonnes of raw sewage in the river each year.

The £4.9bn sewage tunnel, the width of three double decker buses, will lower the volume of untreated sewage mixed with rainwater entering the river from almost 40 million tonnes to 2.5 million tonnes per year so will not reduce it to nothing, she told a meeting of the Environmental Audit Committee.

Water companies dumped untreated sewage into rivers in England 400,000 times last year, Environment Agency figures show, as sewers which combine rainwater and household waste overflow and contaminate the environment.

Not one of Britain’s rivers passes cleanliness tests, with sewage pollution from water companies at least partly responsible for the contamination of half and agricultural pollution the other major cause.

“Currently, when we get inundated with rain, up to 39 million tonnes of rainwater, which then gets contaminated with sewage, is discharged into the tidal Thames, which is clearly unacceptable”, Ms Bentley said. “The Thames Tideway tunnel will eliminate the vast majority of that.