There has been a 7.4% reduction in the number of written complaints, less than the previous year’s 12% drop.

Telephone complaints also fell broadly in line with written complaints.

Billing and charges remains the biggest area of concern for customers, accounting for more than 56% of the total 150,942 written complaints made in the year to 31 March 2013.

The water watchdog said it was concerned that the poorest performing companies still lagged too far behind the rest of the industry on complaint numbers.

It said Southern Water saw the biggest surge in written complaints – a 77% compared to the previous year – making it the worst performing company.

And despite a reduction of nearly 33%, South East Water’s number of complaints remains twice the industry average.

Tony Smith, chief executive of CCWater, 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.”