Six water companies overcharged customers between £800m and £1.5bn by “significantly or systematically” underreporting the true scale of their sewage pollution of rivers and waterways, a tribunal has heard.

In the first environmental competition class action against water companies in England, lawyers argued that the privatised firms had abused their monopoly position to mislead regulators over the amount of sewage they were discharging from their assets over the past 10 years.

As a result, the companies, Thames Water, Yorkshire Water, Anglian Water, Severn Trent, Northumbrian Water and United Utilities, were able to charge customers higher bills than they would have been allowed to if they had provided the regulators with a true picture of their sewage pollution.