Thames Water has been fined £2 million after polluting a stream in Oxfordshire with raw sewage.
Oxford Crown Court ruled the leak, which took place for up to 24 hours on the 8th and the 9th of August 2015, as a high-end, category three harm offence.
The local population of almost 150 bullhead fish in the Idbury brook was killed off by the toxic waste along a 50-metre stretch of water.
The Environment Agency said numerous failures in the management of a sewage pumping station operated by the company were to blame.
The court heard Thames Water disregarded more than 800 high-priority alarms needing attention within four hours in the six weeks before the incident.
Another 300 alarms were not properly investigated, with one being deliberately deactivated during a night shift.
Investigations revealed Thames Water was aware the pumping station failed several times in the year up to the incident in 2015.
Robert Davis, who led the investigation for the Environment Agency, said: “This incident was foreseeable and avoidable.
“Thames Water didn’t recognise the increased risk to the environment, ignoring or failing to respond adequately to more than 1,000 alarms.”
Richard Aylard, External Affairs and Sustainability Director at Thames Water, said: “We take our role in protecting the environment extremely seriously and are really sorry for what happened here in 2015.
“We have made a series of improvements since this regrettable incident, including bringing in more people, more maintenance, more training and better systems. In the three-and-a half-years since, we have not had a serious incident at any of our 4,700 pumping stations.”