Thames Water will have to pay more than £250,000 after the Court of Appeal upheld a ruling against it for allowing untreated sewage to enter a nature reserve.
The court ordered the utility firm to pay a fine of £250,000 and costs of £6,887 in relation to the incident.
In September 2012, untreated sewage entered a nature reserve owned by the National Trust within The North Wessex Downs Area of Outstanding Natural Beauty.
The Environment Agency said Thames Water had failed to act on its alarm system after pumps at the Broad Layings Sewage Pumping Station became blocked. This led to a non-emergency discharge from the pumping station’s overflow pipe, the agency said.
For five days, raw sewage flowed into the reserve, polluting a rare alder carr woodland and a specially developed newt pond, killing invertebrates in watercourses for up to 600m, and closing the reserve to visitors.
Environment Agency deputy director 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.
“Under the new environmental sentencing guideline very large companies who risk causing serious environmental damage could now face very large fines.”
A Thames Water spokesman said: “We very much regret this incident and have since carried out a thorough clean-up of the watercourse and funded an ongoing post for a National Trust warden.
“We have also of course reviewed our procedures to reduce the chance of anything like this happening again.”