Six water companies in England have been accused of overcharging customers between £800mn and £1.5bn by under-reporting the scale of their sewage pollution, in a case that may pave the way for customers to receive hundreds of millions of pounds in refunds.

Competition Appeals Tribunal yesterday lawyers for Carolyn Roberts, a former professor and environmental consultant, accused the privatised companies of abusing their monopoly position to mislead regulators over the amount of sewage they were discharging into rivers since 2015.

Roberts alleges, that, as a result, Thames Water, Yorkshire Water, Anglican Water, Severn Trent, Northumbrian Water and United Utilities, were able to charge  customers higher bills than would have been allowed if they had provided the regulators with a true assessment of their sewage pollution.

The case against water companies in England is being brought under rules introduced by the Consumer Rights  Act 2015, which enabled a single individual to bring   an antitrust claim on behalf of many millions of affected customers.

It is one of the legal challenges faced by water companies and the government as public anger mounts over a combination of storm water and raw sewage pouring into rivers and coastal waters threatening environmental health.

Julian Gregory acting for Roberts told the tribunal that the six companies had misled Ofwat and the Environmental Agency by significantly or systematically under-reporting the number of sewage discharges from their treatment works and combined sewer overflows from 2015 onwards.