The fine imposed on Thames Water for poor service and misreporting was reduced by nearly £3 million when watchdogs decided the firm's processes were so bad it was unaware it had breached rules.
Ofwat said it would cut the penalty from £12.5 million to £9.7 million - 0.7% of Thames Water's annual turnover - having found the group unintentionally misreported regulatory information.
Thames Water will pay £8.3 million for failing to provide adequate information, down from £11.1 million, but its fine for poor customer service remains at £1.4 million, as first announced last September.
Thames Water is one of a raft of water firms to be hit by hefty fines for poor service and compliance.
Severn Trent was fined a record £35.8 million earlier this month, while Southern Water was told to pay £20.3 million for deliberately misreporting information in February.
Ofwat said that it did not find any evidence of purposeful misreporting at Thames Water, but blasted the group for its failings.
Regina Finn, Ofwat chief executive, said: "Thames Water's systems and processes were inadequate and customers lost out. The penalty reflects the importance we attach to reliable, accurate and complete information and gives a clear signal to both the company and the water sector that non-compliance is not a cheap or easy option."
Thames Water welcomed the decision to reduce the fine. It said last year that it would challenge the level of the penalty, which it described at the time as "totally disproportionate".
David Owens, Thames Water chief executive, said: "This is still a high figure considering that the failures were unintentional, rather than deliberate, but we welcome the reduction.
"Although all the misreporting took place under previous ownership and management, we always acknowledged the company should be held to account for these failings, but argued any penalty should be proportionate to the harm done to our customers and the efforts we had taken to make amends."
