Thames Water were fined 125,000 and 21,335 clean up costs last month for polluting a river and killing at least 7,000 fish.
The accident, which occurred in September 2007, happened when the company were cleaning the filters at Beddington sewage works near Mitcham in South London.
Nearly two thousand litres of industrial strength chlorine were accidently released into the river Wandle.
The Environment Agency (EA) told Croydon Crown Court that almost two tonnes, or an estimated 7-8 thousand fish, including barbel, chub, roach, dace, gudgeon and eels were killed along a five kilometre stretch of the river.
This stretch of the river was purportedly one of the best urban coarse fisheries in the country prior to the accident.
It took officials from the EA, helped by local anglers, three days to remove the dead fish from the river which had turned a milky colour and smelled strongly of bleach. Even the green vegetation along the river bed was said to have been bleached white.
Peter Ehmann from the EA is quoted on their website saying "This pollution effectively wiped out 20 years of painstaking restoration work on the River Wandle. For many years individuals and organisations, including the Environment Agency and the Wandle Trust, have achieved great improvements to water quality and aquatic life in the Wandle. This incident is a major set back to all their hard work."
Jonathan Barnard, representing Thames Water, said the company admitted polluting the Wandle and had already pledged 500,000 towards its clean-up and restoration and an additional 10,000 to local angling clubs.
He said: "Thames Water issues an unreserved apology to the local community, to angling clubs and to anyone else affected by this catastrophic incident."
It could take up to 10 years for the river to fully recover and the fish stocks to return to pre- incident standards.
Thames Water was also ordered to pay 21,335 in costs. The total fine represents less than 0.1% of their annual reported turnover of 1.6 billion for 2008.