Levels of consumer complaints about water companies across England and Wales have risen 10% over the past year, according to a watchdog.
Complaints have risen 10% in the past year
United Utilities, Severn Trent, South East Water and Southern all saw their complaint levels rise by more than the industry average.
Incorrect billing and poor complaint handling were the chief gripes, the Consumer Council for Water (CCW) said.
In total, the CCW received 13,603 complaints in the year to March.
"It is particularly disappointing to find some companies, for the second consecutive year, tripping up when they have introduced basic billing systems - causing customers great concern," said Dame Yve Buckland, chair of the CCW.
"In some cases these problems are compounded when customers cannot even reach their company's staff on the phone to complain," Dame Buckland added.
Despite being singled out as one of the worst performers, Brian Hurd, customer services director of United Utilities, said complaint levels had started to fall at his company since last November.
"The majority of the complaints occurred in the first half of the year when we were still in the process of bedding in a new computer billing system," he explained.
In a statement, Severn Trent water said that they were "disappointed to see an increase in complaints" and were "determined" to tackle the issues causing concern to their customers.
Other water companies performed better and some saw a fall in the number of complaints for the whole of 2006/2007, compared to the previous 12-month period.
At South West Water complaints fell by 15%, though it still attracted most complaints as a proportion of its total number of customers.
Wessex Water and Thames water saw complaints fall 13% and 9% respectively.