Thames Water allowed untreated sewage to enter Chase Brook which flows through a 143 acre, National Trust owned nature reserve within an area of outstanding natural beauty and a site of special scientific interest in September 2012.
The pollution event was brought to the attention of the Environment Agency and the fine was issued to Thames Water in August 2014, but the water company appealed, claiming it had been harshly treated.
The Court of Appeal ruled that the fine, and £6,887 of costs to be paid, should stand, adding that it would have also upheld a “substantially higher fine in this case”.
The Environment Agency’s deputy director (chief prosecutor) of legal services Anne Brosnan said: “We welcome the Court of Appeal’s decision to uphold this significant prosecution result, which demonstrates that businesses need to prevent pollution or their profits could take a hit.”
She added: “In this instance the company did not act swiftly enough to stop sewage damaging the nature reserve and a significant clean-up operation was needed. This sentence should act as a deterrent.”
The pollution incident occurred after two pumps at the Broad Layings Sewage Pumping Station failed. This followed 16 failures of one or both of the pumps in the previous five months.
Alarms at the Thames Water control centre sounded more than a dozen times warning of rising sewage levels, but it was five days before the company attended the site.
During this time sewage flowed into the nature reserve polluting a rare alder carr woodland and a specially developed newt pond, killing invertebrates in watercourses for up to 600 metres, and closing the reserve to visitors.
Thames Water has since replaced the pumps with newer more robust units and added it “very much regretted” the pollution incident.