Thames Water was fined after pumping sewage into a water tanker in Mortlake without the permission of Richmond Council.
The company admitted the offence at Lavender Hill Magistrates’ Court this month, as well as not providing adequate signs to ensure the works were carried out safely.
It was fined £1,500 for working without a permit and £1,000 for the lack of signs.
Councillor Pamela Fleming, council cabinet member for environment, said: “This prosecution could have been avoided with some forewarning and more care and attention.”
A Thames Water spokesman said: “Tankers are used every day across our region to protect homes from sewer flooding, and we place trust in our teams to make sure they get the appropriate permissions.
“Unfortunately we were let down in this instance but we are working with our contractors to make sure this doesn’t happen again.”
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