Thames Water has been fined a record £20m ($24.9m, €23.1m) for dumping raw sewage into the River Thames.
The utility firm admitted six counts of water pollution and other offences at its facilities in Buckinghamshire and Oxfordshire, which poured 1.4 billion litres of effluent into the river.
It is the largest penalty handed to a water utility company for an environmental disaster in the UK.
The spills resulted in hundreds of dead fish and birds, overflowing manholes and sewage spilling into nature reserves, leaving farmers with sick animals. The offences took place between 2013 and 2014.
Environmental damage was caused in the riverside towns of Henley and Marlow.
There were also reports of nappies and other sewage debris spilling into the Thames.
The previous record fine for any water company is £2m paid by Southern Water for an incident in Margate, Kent, over Jubilee Weekend in 2012. The previous record fine paid by Thames Water was the £1m it forked out in January 2016.
Judge Francis Sheridan, at Aylesbury Crown Court, said customers should not be punished with higher bills. He said the fine should be borne by the group's shareholders.
Judge Sheridan condemned the "disgraceful conduct" of Thames Water adding the spillage was "entirely foreseeable and preventable."
The case against Thames was brought by the Environment Agency. The agency's chief prosecutor Anne Brosnan said: "Thames Water was completely negligent to the environmental dangers created by the parlous state of its works.
"Our investigation revealed that we were dealing with a pattern of unprecedented pollution incidents which could have been avoided if Thames Water had been open and frank with the Environment Agency as required by water company industry protocol."
Chief executive of the Environment Agency Sir James Bevan said when utilities "experience problems through no fault of their own we will always work with them to resolve them, but where negligence causes serious pollution, or a serious threat to the environment, we will seek the strongest possible penalties".
Thames Water chief executive Steve Robertson, who was appointed last September after the spill, said: "We deeply regret each of these incidents at six of our sites during the period 2012-14."
He added: "It was clear that our performance in this part of our region, at that time, was not up to the very high standards that we and our customers expect."
The group said since the spill it had increased staff numbers in key operational roles at the sites and invested heavily to improve reliability.
Robertson said: "As a result, our performance has significantly improved. We understand our huge responsibilities to the environment, have learned from these serious events, and continue to invest at the rate of around £20 million a week on continually improving our service to our customers and the environment."
River Thame Conservation Trust chief executive Louise Bowe added: "We have been shocked as the catalogue of events from 2013 have come out in court. It was much worse than we thought and explains how the devastating damage to the river came about.
"Some large fish that were killed cannot be replaced as species like large mature chub and roach are simply not available."
Thames Water serves 15 million customers, operates 350 sewage works across London and the Thames Valley and is responsible for 68,000 miles of sewer pipes.