Water bills are expected to change slightly in the coming years
The number of vulnerable people seeking help to pay their water bills in England has risen 20% in the past year, figures from the regulator Ofwat show.
The data also reveals that the take-up of the WaterSure tariff varies widely in different areas of England.
Every water company in England must offer the tariff to those households on certain benefits and with children, or those housing people who are ill.
Ofwat is to investigate how suppliers deal with vulnerable customers.
The WaterSure tariff was formerly known as the Vulnerable Groups Scheme and was introduced in 2000.
The payment system is open to people on water meters who receive benefits or tax credits and have responsibility for at least three children aged under 19.
It is also available to households where somebody has a medical condition that means they need more water, such as Crohn's disease or Eczema.
Those in the scheme will pay no more than the average household bill for their region.
Applications must be made every year and they increased from 24,121 in 2007/08 to 28,879 the following year. The numbers have grown year-on-year from 4,171 in 2000/1.
However, an estimated 100,000 customers could take advantage of the tariff and a more detailed breakdown of the figures shows how some areas of England see a much higher take-up than others.
There were 6,782 applications to South West Water for the tariff, many more than the next highest take-up of 3,399 Severn Trent customers.
Although the size of the companies' customer base, the proportion of customers who have meters, and household income all affect this take-up, Ofwat chief executive Regina Finn said promotion of the scheme was an issue.
"Unfortunately there are still many people out there who could benefit from the tariff who do not know about it," she said.
The regulator sets guidelines on how companies deal with vulnerable customers and a workshop is being held next year to encourage companies to communicate effectively with these groups of people.
The WaterSure tariff is advertised in a number of ways, including information on bills, leaflets in GP's surgeries and libraries, as well as talks to community groups and charities.
The two water companies operating in Wales have similar tariffs, but only operate on a voluntary basis.
In July, Ofwat revealed how much companies could charge for water and sewerage in 2010 to 2015.
In a snub to the companies, it said the typical bill in England and Wales should be cut by £14 to £330 over the five years, excluding inflation, although the companies had wanted prices to rise.
But companies suggest that customers might suffer as a result of the plans.
Water and sewerage services in Scotland and Northern Ireland are regulated separately.