Thames Water has been handed a £3.3 million fine for a “reckless” incident in which “millions of litres” of undiluted sewage was pumped into rivers near Gatwick Airport in 2017.

A two-day sentencing hearing at Lewes Crown Court was told there was a “significant and lengthy” period of polluting the Gatwick Stream and River Mole between Crawley in West Sussex and Horley in Surrey on October 11 2017.

Judge Christine Laing KC said on Tuesday that she believed Thames Water had shown a “deliberate attempt” to mislead the Environment Agency over the incident, such as by omitting water readings and submitting a report to the regulator denying responsibility.

Thames Water had pleaded guilty on February 28 to four charges relating to illegally discharging waste in October 2017, but had denied seeking to mislead the Environment Agency in the events, instead arguing that “significant errors were made”.

The court heard that a storm pump unexpectedly activated and was filling up the storm tank, despite no substantial rainfall, for 21 hours, which went unnoticed.

On October 11 2017, the pump then began spilling the sewage into the river for an estimated six hours.

The court heard that no specific alarms to alert staff to the overspill, or how long the undiluted sewage was entering the tank, were in place.

When an alarm was heard, the lead technician was uncontactable as they were awaiting a new mobile phone.

Judge Laing KC said she found it “utterly extraordinary” that environmental disasters can occur because of issues such as this.

Addressing the UK’s largest water company, Judge Laing KC spoke of how “extremely serious” the number of previous convictions they had was, adding: “They should have put in every effort into tidying up the problem areas.”

The judge also criticised Thames Water for the 2017 incident taking place just months after the firm was fined a then-record £20 million in March 2017 for a series of pollution incidents on the River Thames.

The penalty comes as the utility giant, which serves 15 million households across London and Thames Valley, faces concerns over its future amid mounting 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.

The current record fine against a water company for illegal discharge of sewage is held by Southern Water at £90 million for nearly 7,000 incidents across Hampshire, Kent and Sussex in a case brought by the Environment Agency in 2021.

Alongside the £3.34m fine, Thames Water has been ordered to pay Environment Agency costs of nearly £129,000.

This latest prosecution means Thames Water has paid £35.7 million for pollution incidents between 2017 and 2023.

Speaking after the sentencing, Jamie Lloyd, who led the Environment Agency investigation, said Thames Water “missed several opportunities” to prevent the pollution from happening and did not provide “vital information” when requested by the regulator.

The senior environment officer said: “We brought this case due to the major environmental impact caused, and because it was entirely avoidable.

“Thames Water failed to have adequate systems in place to manage the pollution risk from their site and didn’t respond to alarms.”

After the sentencing,  a statement from Thames Water’s interim co-chief executive, Cathryn Ross, said the company was “deeply sorry” for the “unacceptable pollution”.

Ms Ross said: “I would also like to express my sincere apologies for those aspects of our response to the incident six years ago that led to the finding that we misled the regulator.

“We fully accept that we made significant errors and exercised poor judgment at the time, and we are genuinely sorry for that.”

The water company boss also highlighted the £33 million plan to improve the works at Crawley Sewage Treatment Works, and is introducing upgrades at other sites.

A voluntary payment of £1 million was also given to local organisations to try to “make up for the harm caused” by funding projects such as fish passage and habitat works.

Ms Ross added: “We have undertaken a full evaluation of this unacceptable incident and have learned the lessons.”