Thames Water has been fined £3.3m after it discharged millions of litres of undiluted sewage into two rivers, killing more than 1,400 fish.
There was a “significant and lengthy” release of sewage from treatment works near Gatwick Airport in October 2017 into the Gatwick Stream in Sussex and River Mole in Surrey, a court heard.
Thames Water admitted four charges in an Environment Agency prosecution.
It was handed the fine during sentencing at Lewes Crown Court.
The company, which serves 15 million households, faces concerns over its future amid mounting debt.
Judge Christine Laing KC said she believed Thames Water had shown a “deliberate attempt” to mislead the Environment Agency, by omitting water readings and submitting a report to the regulator denying responsibility.
The court heard how a storm pump unexpectedly activated and was filling up the storm tank, despite no substantial rainfall, for 21 hours, which went unnoticed.
The court heard the pump then spilled sewage into the river for an estimated six hours but no alarm was in place to alert staff to the overspill.
When an alarm was heard, the lead technician was uncontactable because they were awaiting a new mobile phone, the court was told.
During the hearing, the court had been told by Sailesh Mehta, prosecuting, the sewage spill was an “accident waiting to happen”.
Defending Thames Water, Lisa Roberts KC claimed there was a “faulty switch” in the storm pump that caused the incident which “could not have reasonably been foreseen”.
She said: “It shouldn’t have happened and Thames deeply regrets the event.”
The judge said she found it “utterly extraordinary” that environmental disasters could occur because of issues such as this.
She said the company “should have put in every effort into tidying up the problem areas”.
Thames Water has had 20 previous fines for pollution spillage.
The company’s chief executive, Sarah Bentley, stepped down last week after two years in the role, weeks after giving up her bonus over sewage spills.
The record fine against a water company for illegal discharge of sewage is held by Southern Water at £90m.
That fine followed nearly 7,000 incidents across Hampshire, Kent and Sussex in a case brought by the Environment Agency in 2021.
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