Southern Water has been fined £20.3 million by watchdog Ofwat for poor customer service and deliberately misreporting information.
The company said shareholders rather than customers would foot the bill.
Ofwat chief executive, Regina Finn, said: "Southern Water behaved unacceptably in deliberately misreporting customer service performance to Ofwat and systematically manipulating information to conceal its true performance over an extended period of time and the company has acknowledged this."
Accurate information is important because it is designed to protect consumers without a choice of supplier, according to Ofwat.
Southern Water said the problems came to light after a new billing system was installed, which showed the customer service response times reported to Ofwat did not always match performance.
Southern Water chief executive, Les Dawson, said: "The new management team has put procedures into place to ensure this will never happen again and once again we apologise to our customers.
"I can also assure them the fine will be borne by the company's current shareholders even though neither they, nor our previous owners, knew of the deception that took place before the company's present management team was appointed."
In addition, Southern Water has agreed to reduce future customer bills which were higher than they should have been as a result of the misreporting.
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