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.

In a statement released after the verdict, interim co-CEO of Thames Water Cathryn Ross said: "We are deeply sorry for the entirely unacceptable pollution incident into the Gatwick Stream and River Mole six years ago.

"The incident occurred due to the running of a storm pump in error. The pump activated when there was no operational need for it to do so. This had never happened on this site before and it has not happened since," she said.

"It should not have happened, and we deeply regret the incident. 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.

"To make up for the harm caused, so far as it is possible, we have made voluntary payments totalling £1m to three local organisations to fund projects including the development of a local catchment plan and carrying out fish passage and habitat works."

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.