An increasing number of households are not paying their water bill, new figures show.
Ofwat said there had been a 17% increase in unpaid bills between 2010 and 2015 in England and Wales despite the number of customers at risk of struggling to pay remaining stable.
Some £2.2 billion is currently outstanding, and unpaid bills now add around £21 to the annual charges for every household - an increase from £17 in 2009/10, Ofwat said.
The report shows that low-income households, working-age adults living alone, lone parents and single pensioners are more likely to have problems paying and are more likely to be in debt.
Ofwat said most companies had not achieved their initial targets for customers signing up to new social tariffs, which offer cheaper bills to those who are struggling.
The proportion of customers contacting National Debtline with issues related to water bills has increased by almost 10% in the last five years.
The increase in debt over the last five years comes despite below-inflation rises in average bills.
Ofwat chief executive Cathryn Ross said some companies needed to improve at collecting money from those who choose not to pay, but said it was clear that many households needed support.
She said: "This important research highlights that while three quarters of customers think their water services are good value there is a growing problem that more customers are not paying their bill.
"Some companies need to get better at collecting unpaid bills from those customers who can pay but don't. Yet this report is clear that many households are feeling the squeeze and need support.
"While some companies are improving how they respond to these customers, others have to get better at identifying those at risk and helping those who are genuinely struggling.
"This is not only the right thing to do, but benefits everyone as it will bring down the cost to everyone of unpaid bills and increase trust and confidence in the provision of these vital public services."