Two tonnes of dead fish had to be sifted from the River Wandle after Thames Water spilled industrial-strength bleach into the waterway, a court heard.
The water company faces a severe fine after pleading guilty to inadvertently releasing 1,600 litres of sodium hypochlorite into the river in September last year at Sutton Magistrates' Court on Friday.
Prosecutor Janet Fedrick, acting on behalf of the Environment Agency (EA), said it was the most serious case of river polluting she had seen brought before a court.
“The effect of this was catastrophic with more than 10,000 fish killed, and five kilometres of the river polluted,” she told the court.
“To anglers and members of the public the amenity value of the river has been seriously diminished and it’s believed it will take several years to recover.”
Janet Fredick added increased sinage and padlocks at the Beddington works were put in place after the accident and before EA officers inspected the sewage works.
She said: “Clearly they tried to lock the stable door after the horse had bolted.”
A Thames Water lawyer was unable to convince magistrates to impose their maximum penalty of £20,000.
Instead the case has been taken to Croydon Crown Court where a judge can impose an unlimited fine.
A mechanical fault at Beddington Sewage Works allowed the chemical, similar to that used in swimming pools, to flow into the Wandle.
But the court heard that employees, who were cleaning the sewage works at the time, did not think the spill serious enough to alert the Environment Agency.
Several miles of the river had turned white within hours, dead fish were seen floating in the river, and two police officers sent to investigate found the smell of bleach so strong that they felt nauseous.
Unsuccessfully arguing for the lowest possible sentence, Defence lawyer Jonathan Barnard said that Thames Water had already paid the entire cost of cleaning the river.
He added that £500,000 had been pledged by the water company to help continue restoration and cleaning work on the river over the next five years.
But magistrates took just 20 minutes to reach a decision that the severity of the crime exceeded their sentencing powers.
They had earlier been shown multiple photos highlighting the environmental damage caused by the spill.