Customer complaints have increased at almost half of all water suppliers serving England and Wales, with gripes over billing and charges accounted for over 60% of all complaints across the industry, according to a consumer body.
Southern Water, Dwr Cymru Welsh Water, Bournemouth Water and Affinity Water (which serves parts of Central, East and Southeast England) were singled out for their poor performance in 2015-16 by government-backed body the Consumer Council for Water (CC Water).
Those four firms will have to report back to CC Water next month on how they're reducing complaints levels.
Southern Water has extended its four-year reign as the most complained about water company, with 77 written complaints per 10,000 connections - up 10.3% since 2014-15.
However Welsh supplier Dwr Cymru saw its written complaints more than double, while Bournemouth Water and Affinity Water have seen increases, of 90.6% and 82.3% respectively.
Not all suppliers performed badly. Wessex Water had 13 written complaints per 10,000 connections while Portsmouth Water had just 8.7.
CC Water's report comes two days after industry regulator Ofwat claimed removing water firms' regional monopolies and allowing customers to switch suppliers would have "financial and service benefits."
What does the industry say?
Southern Water director Simon Oates admitted there was "still much more for us to do to come in line with the rest of the industry".
He said: "We want to reassure our customers we remain fully committed to continuing the improvements we have already made, and bringing our performance up to the standard they quite rightly expect from us.
"Our performance since November 2015 - most of which unfortunately falls outside the time period covered by today's report - has been much improved."
CC Water chief executive Tony Smith said: "What is particularly alarming is the very substantial increase in complaints some companies have reported.
"We can't allow the good progress that has been made by the industry in recent years to be reversed. That's why we have asked four of the poorest performing companies to report to us by the end of October on what action they have taken, or are planning to take, to reduce complaints."
Additional reporting by Press Association.