For the fourth year in a row, Southern Water has been revealed as the worst-performing company, receiving double the number of complaints than the industry average.
Nearly half of water companies across England and Wales saw an increase in written customer complaints in 2015-16, with some recording "alarming" surges, a watchdog said.
The Consumer Council for Water (CCWater), which released the figures, has asked four companies to report back by the end of October to explain what they are doing to drive down complaints.
The four companies are Southern Water, Dwr Cymru Welsh Water, Bournemouth Water and Affinity Water.
Across the industry, billing and charges were the biggest areas of complaint, accounting for just over 60% of complaints to companies in 2015-16. This category includes liability for charges, prices and debt recovery.
For the fourth year in a row, Southern Water was the worst-performing company, having seen a 10.3% increase in complaints compared with 2014-15. Southern received around 77 written complaints per 10,000 connections from customers - more than double the industry average of 33.7.
Written complaints to Bournemouth Water and Affinity Water reported large increases, of 90.6% and 82.3% respectively.
CCWater said the gap between Southern and the rest of the industry is widening.
It said: "The company has a lot of work to do to close that gap. We expect the company to do so."
Southern Water said in a statement that billing complaints pushed it into the bottom spot overall, but pointed out that 2015/16 was its best ever year in terms of operational performance.
Our performance since November 2015 – most of which unfortunately falls outside the time period covered by today’s report – has been much improved. We have seen the number of complaints we receive from customers fall steadily and, so far this year, we have received approximately half the number of complaints we did in the first five months of 2015.
The last year saw more companies reporting an increase in complaints than in any year since 2008-09.
CCWater is a Government-sponsored public body independent of water companies and regulators.
It said that at the other end of the spectrum, South East Water reported the largest reduction in complaints (down by 38.2%), with other companies also reporting large falls including South Staffordshire Water, Severn Trent Water, Bristol Water and Thames Water.
Wessex Water was named as the best-performing water and sewerage company with a 19.2% reduction in written complaints. Portsmouth Water, which saw a 18.9% fall in complaints was named as the the leading water-only company.