Thames Water has been ordered to pay £380,000 for polluting a rural stream in Buckinghamshire.
According to the Environment Agency, the fine follows “numerous failures” reported at Princes Risborough sewage treatment works between February and July 2013.
Traces of sewage were found up to 4km downstream, killing wildlife, it added.
Environment Officer, Holly Linham said: “Our officers attended the site where they walked the Horsenden Stream and saw thick layers of sewage fungus carpeting the river bed and plants. One of the officers had never seen sewage fungus in a stream that bad before. A biological survey of the stream noted that the impact of the sewage was chronic and was likely to have been prevalent for some time.
“The conditions observed by Environment Agency officers during their visits were not isolated incidents. Log book entries suggest ongoing discharges and other problems at a site that was struggling to cope.”
Thames Water said it takes its responsibilities “extremely seriously” and very much regrets the incident.
A spokesperson added: “We have since made a number of changes at Princes Risborough sewage works to further improve resilience and protect the Horsenden Stream.
“This has included raising the level of the storm weir to prevent unnecessary discharge into the stream and restructuring how the site is managed to ensure it is better maintained.”