More than half of all complaints to water companies are about billing and charges.
They accounted for almost 60% of the 106,000 customer complaints made in England and Wales in the last year, according to Consumer Council for Water (CCWater).
However the number of complaints to water firms have fallen for a seventh successive year, it revealed.
Despite that, the gap between the industry’s best and worst performers remains “unacceptable”, CCWater added.
Southern Water remained the worst performing company for the third year running, with complaint levels more than twice the industry average despite an almost 13% reduction in 2014/15.
Anglian Water, Affinity Water, Portsmouth Water and Hartlepool Water reported an increase in complaints compared to the pervious year.
Tony Smith, Chief Executive of the Consumer Council for Water, said: “We expect the water industry to continually try and improve its services to customers and many water companies deserve credit for rising to that challenge year after year.
“However the gulf between the best and worst performers remains unacceptable.”