Written customer complaints against Southern Water have risen by 77% on the previous year, making it the worst performing company in England and Wales.
The failings were announced today by watchdog the Consumer Council for Water in its annual written complaints report.
In the year 2012/13, Southern Water received 22,815 complaints - up from 12,863 on the previous 12 months.
Southern Water and South East Water's poor complaints record has been revealed
Of the 10 companies that deal with water supply and sewage in England and Wales, only Yorkshire Water also saw a rise in complaints.
In a statement on its website, Southern Water admitted it received more telephone calls from its customers than it could deal with effectively, which resulted in an increase in written complaints.
The CCWater report also found South East Water's number of complaints remains twice the industry average, despite a reduction of nearly 33% in the last year.
It ranked worst among the 13 companies who deal only in drinking water in England and Wales, receiving 8,787 complaints.
“Put simply, our service has not been good enough and these figures are unacceptable – but we’re doing something about it..." - Southern Water's Darren Bentham
The next worst performing firm, Essex and Suffolk Water, received 2,717.
Southern Water's chief customer officer Darren Bentham said: "I'd like to apologise for letting our customers down.
"Put simply, our service has not been good enough and these figures are unacceptable – but we're doing something about it.
"We've carried out a major review of almost every aspect of our work with customers and have made some key changes, including putting a new management team in place.
"Things are getting better – we have more staff on the phones covering longer hours and we are receiving fewer complaints.
"We're not the finished article but we've built up some great momentum."
CCWater chief executive Tony Smith said: "We are concerned the rate at which complaints have fallen has slowed and the poorest performers remain too far behind the rest of the industry.
"CCWater continues to meet regularly with water companies to put pressure on them to prioritise customer service in order to see their complaint levels drop much further.
"We also continue to tell the regulator Ofwat that there needs to be stronger penalties for water companies who are failing to satisfy the expectations of their customers."
CCWater London and south east chairman Sir Tony Redmond said: "There are some signs this year that things are improving for both Southern Water and South East Water.
"But given the problems that particularly Southern Water has had in the last year, we need a very significant improvement to give CCWater some sort of comfort that this is being tackled properly."
Southern Water supplies water and manages sewage in Kent and parts of Sussex, Hampshire and the Isle of Wight.
South East Water also supplies drinking water to customers in parts of Kent, Sussex, Surrey, Hampshire and Berkshire.
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