Thames water, which supplies 15m people in london and the south-east, has suffered a surge in written complaints, making it englands worst-performing water company for the second year in a row.
Written complaints to the company increased 57 per cent to 33,727 in the 12 months to the end of march.
Across the industry, household written complaints reached their highest level in four years at 84,649, according to the annual report from the consumer council for water.
The overall figure was pushed higher by thames water as two-thirds of suppliers experienced falling complaints.
Southern water, supplying hampshire, sussex and kent, was the second poorest performer with a 22 per cent rise to 5,594 in written complaints.
Emma clancy, chief executive of the consumer council for water, said: consumers expectations of their water company are very simple they want accurate, affordable bills and a service they can always rely on but some suppliers are still not getting the basics right.
Nearly two-thirds of all complaints relate to billing and charges, including 24,100 complaints to thames water, the largest supplier of water and sewage services in england.
The company has cited the move to a new online billing system and an it issue in the final quarter of the year for causing the increase.
But thames was also the worst performer for complaints over water supply, receiving 67 per cent more complaints per 10,000 connections than the next worst performing company.
Kelly macfarlane, thames water customer experience director, said: we recognise this is not acceptable and we are not happy with our performance last year. we always aim to provide great service for our customers and were absolutely determined to improve our performance.
Written complaints only tell part of the story, accounting for just 11 per cent of grievances overall.
The consumer watchdog only started to collect data on telephone complaints in the last six months of 2019/20 so will provide full comparisons next year, it said.
Companies received 341,307 telephone calls to complain about services in the last six months of the year.
Although the vast majority of complaints are made by telephone, the watchdog said its focus on written complaints was justified as it indicates a degree of seriousness.
Where a consumer is unhappy enough to have raised their grievance in writing, this suggests that they are among the most dissatisfied complainants, it said.
The watchdog was particularly concerned about a 30 per cent increase in the number of customers that had to write to water companies a second time because their complaints had not been handled.