Poor customer care reported by Severn Trent and other water companies has undermined confidence in the industry, the regulator said yesterday.
An Ofwat report says firms in England and Wales generally provide "good" levels of service.
Extra investment has cut the risk of internal sewer flooding and reduced the number of buildings with low water pressure. But the overall picture of industry performance in handling complaints, billing issues and phone calls was affected by the "poor performance" of three companies, the report says. Those firms - Severn Trent, Southern and Thames Water - are each currently subject to Ofwat investigations.
The regulator has already said it intends to fine the three companies, but it cannot say by how much until its investigations are complete.
Ofwat chairman Philip Fletcher said: "These disappointing cases have undermined customers' confidence in the industry and raised concerns about the reliability of the data that the companies report to us."
The Levels of Service for the Water Industry in England and Wales 2005-06 report reviews the performance of 12 water companies and ten water and sewerage companies.
A Consumer Council for Water spokesman said most companies were putting in a good performance across the board.
"However, we're extremely disappointed that three large companies - Southern, Severn Trent and Thames - could not provide reliable enough data for their performance to be properly recorded in the report," he said.
"Reliable data is essential to protect consumers' interests. Company data to Ofwat shapes the future of the bills that we all pay, and the levels of service that we can expect from the water industry."
The three water companies which are subject to Ofwat investigations have all pledged to reimburse customers for any disadvantage caused.