Thames Water have been fined more than £3m after admitting polluting rivers.
The company, which supplies one in four people in Britain with water, had pleaded guilty to four charges relating to illegally discharging waste.
It was fined £3.3m at Lewes Crown Court on Tuesday.
The court heard “millions of litres” of undiluted sewage was pumped into the Gatwick Stream and River Mole between Crawley in West Sussex and Horley in Surrey on 11 October, 2017.
The hearing was told that the spill turned the water “black” and killed more than 1,000 fish.
Judge Christine Laing KC said that she believed the firm had shown a “deliberate attempt” to mislead the Environment Agency over the incident, by omitting water readings and submitting a report to the regulator denying responsibility.
The company has previously been fined £32.4m for pollution incidents in the Thames Valley and south-west London between 2017 and 2021.
During the first day of the hearing on Monday, the court heard how a storm pump at Crawley Sewage Treatment Works site was unexpectedly diverting sewage to its storm tank for 21 hours and went “unnoticed”.
Prosecutor Sailesh Mehta estimated untreated sewage was spilling into the river for six and a half hours after no alarm was raised.
When an alarm was raised the lead technician was unreachable as they were waiting for a new mobile phone.
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Eyewitness accounts read in court said how they saw the river turn “black” and “grey”, with “huge numbers of dead fish” visible in the water.
Nearly 1,400 dead fish were recovered from the rivers by the Environment Agency following the incident.
Lisa Roberts KC, representing Thames Water, said the firm expresses its “unreserved and sincere apology” for the incident, adding: “Put bluntly, it shouldn’t have happened and Thames deeply regrets the event.”
She said the company rejects that previous issues were to blame for the spillage, putting it down to a “faulty switch” in the storm pump which meant the incident could not have been predicted.
A £33m plan to improve the Crawley site has been put in place since the incident, according to Ms Roberts, with aims to complete it by the end of March 2025.
New systems have also been rolled out across other Thames Water sites to prevent such incidents happening again.
The fine comes as the company faces concerns over its future amid a mounting £14bn debt.
Thames Water’s chief executive Sarah Bentley stepped down with immediate effect last week after she gave up her bonus due to the company’s environmental performance.
In 2021, Southern Water was fined a record £90m for nearly 7,000 incidents of illegal discharge of sewage across Hampshire, Kent and Sussex.