The UK's largest water company has apologised after it was fined £2.3m for polluting a stream with sewage that killed more than 1,000 fish.
Thames Water pleaded guilty at Aylesbury Crown Court after sewage with high levels of ammonia was found by the Environment Agency (EA) in Fawley Court Ditch at Henley-on-Thames in 2016.
The agency said the stream lost "almost all its fish to the pollution".
It added Thames Water has been fined £24.4m for similar offences since 2017.
The latest incident is the ninth of it is kind across Oxfordshire, Berkshire and Buckinghamshire, the EA said.
The agency said the "poor management" at Thames Water's treatment plant was "laid bare" at the court hearing, after its investigation found ammonia levels in Fawley Court Ditch were "at worst, double the permitted limits".
It said machines to reduce ammonia totals were not working and officers found oxygen at the plant that helps control the treatment was "dangerously low 24 hours before the incident".
Jackie Outhwaite, a land and water officer for the agency, said Thames Water "could and should have prevented this pollution through better management of sewage treatment".
She said: "Our investigation found the risk of pollution was increased by a lack of measures in place to prevent it.
"Thames Water's failure to respond to warning alarms ultimately led to significant harm on water quality."
Fish from 13 species died and the stream took a year to recover, the EA said.
Ms Outwaite added the "damage caused to the environment at Henley" showed "water companies have a lot more to do to protect the environment".
Thames Water pleaded guilty to one count of causing a discharge of partially-treated effluent into Fawley Court Ditch and Fawley Court Stream without an environmental permit between 21 and 24 April 2016.
Sarah Bentley, Thames Water CEO, said the company was "really sorry for what happened in Henley".
"Discharges of untreated sewage are simply unacceptable and we will work with the government, Ofwat and the Environment Agency to accelerate work to stop them being necessary," she added.