After dumping millions of liters of undiluted sewage into two rivers, killing more than 1,400 fish, Thames Water was fined £3.3 million.
In October 2017, the company discharged sewage from treatment facilities close to Gatwick Airport into the Gatwick Stream in Sussex and the River Mole in Surrey.
Four charges were made against Thames Water in an Environment Agency case.
It received the fine at Lewes Crown Court's sentencing.
The company, which provides services to 15 million households, is worried about its financial future due to rising debt.
Sailesh Mehta, the prosecutor, told the court during the hearing that the sewage spill was "an accident waiting to happen.".
Lisa Roberts KC, defending Thames Water, said: "Thames deeply regrets the incident. It shouldn't have happened. ".
She insisted that the incident, which "could not have reasonably been foreseen," was brought on by a "faulty switch" in the storm pump.
