A water company has apologised and vowed to improve after it was named the worst performing in the country.

Complaints against Southern Water soared by 77% in a year, resulting in the highest number of complaints per customer across the UK and bucking the national trend of falling complaints against water companies.

The firm, which made a profit after tax of £156.9 million last financial year, has apologised and promised to improve its service.

South East Water also came under fire for having double the industry standard number of complaints, despite them dropping by a third in the past year.

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According to the Consumer Council For Water report, Southern Water received almost 23,000 written complaints in 2012/13 while South East Water had more than 8,700. Almost two-thirds of complaints to Worthing-based Southern and three out of five to South East were about charging or billing.

The pair had the highest number of complaints per customer in the country at more than double the industry average of 49 complaints for every 10,000 connections.

The report said the increase in complaints to Southern was because of “inadequate staffing resources” and customers’ dissatisfaction about receiving the highest increase in water and sewerage charges.

In February, Southern Water announced that customers could expect their bills to rise by 5.3% on average.

The company has been told by the council it needs to do “a lot more” in order to improve its service and reduce its complaints.

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’re not the finished article but we’ve built up some great momentum.”

The company said it carried out a major review and, following changes, the number of written complaints in June, July and August this year had dropped by 40% compared with last year.

The firm also said in 2012/13 it received more customer phone calls than it could deal with effectively, which resulted in an increase in written complaints, but that it had now extended its call centre hours.

Steve George, customer services director for South East Water, said: “Although we are pleased with our improvement our customers rightly expect an even better service and we’re working really hard to deliver this.

“We are grateful to the Consumer Council for Water whose staff have worked closely with us to help review our processes and offer us constructive feedback to ensure we keep driving customer service improvements.”