Thames Water’s appeal against a £250,000 fine for polluting a nature reserve has been upheld.

The company allowed untreated sewage to enter a 143 acre nature reserve in September last year.

The sentencing decision took into account the profit made by the company to ensure the fine was “proportionate”.

Despite repeated alarms in Thames Water’s control centre showing a total pump failure at a sewage pumping station, the company didn’t attend the site for five days said the Environment Agency.

The raw sewage flowed into the reserve polluting a rare woodland and a specially developed newt pond, killing invertebrates in watercourses for up to 600 metres and closing the reserve to visitors.

The Court of Appeal said: “To bring the message home to the directors and shareholders of organisations which have offended negligently more than once before, a substantial increase in the level of fines, sufficient to have a material impact on the finances of the company as a whole, will ordinarily be appropriate.

“This may therefore result in fines measured in millions of pounds.”

A Thames Water spokesperson 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.”