The water regulator Ofwat has serious concerns over the way sewage treatment works at five water companies have been operated, as it continues to investigate the industry.

Anglian Water, Northumbrian Water, Thames Water, Wessex Water and Yorkshire Water submitted reports in December that the regulator, Ofwat, said were worrying. It said on Wednesday it had started enforcement cases against all five companies.

In 2020 the Guardian revealed that water firms had discharged raw sewage into England’s rivers 200,000 times the previous year. And last year Ofwat and the Environment Agency announced an investigation into potentially illegal dumping of raw sewage into rivers and coastal waters by water companies.

The inquiry came after evidence that the scale of illegal discharges of raw sewage could be 10 times higher than disclosed by the firms. As part of the inquiry, water companies had to disclose information about the way they operated their treatment works, including details of potentially illegal discharges into waterways.

“The data that emerged at the end of last year suggested widespread shortcomings in how water companies were running sewage treatment works. The first phase of our investigation suggests those concerns are credible,” said Ofwat’s interim head, David Black.

“We have identified shortcomings in most water and wastewater companies and are continuing to investigate. But we have already seen enough in five companies to cause serious concern and warrant us taking further action.”

Ofwat said companies either reported a large number of wastewater treatment works that may not be meeting environmental rules, or failed to show how they were operating their works, or it had concerns about how companies met environmental obligations.

“We will now dig deeper into what these five companies have been doing, with the prospect of formal enforcement against them if we find they are failing on obligations Ofwat enforces,” Black said.

“We will have further questions for all companies on this. In the meantime, we expect them to make quick progress in addressing any potential non-compliance they might have, whilst strengthening how they manage their environmental obligations as a whole.”

Earlier this year MPs on the environmental audit committee called for more assertive regulation of water companies as they said England’s rivers had become a “chemical cocktail” of sewage, agricultural waste and plastics.

MPs want to see an urgent review of the way water companies are allowed to self-report pollution, and are alarmed at the extent of sewage discharge, large spills by water companies and misreporting.

Southern Water was given a record £90m fine last year for discharging billions of liters of raw sewage into coastal waters off Kent and Hampshire over a five-year period.