All 11 water firms in England and Wales are under scrutiny by Ofwat as part of its major investigation into whether their sewage treatment works are polluting the environment.

The water regulator said it showed “how concerned we are about the sector’s environmental performance”.

Ofwat announced it had served formal enforcement notices on the four remaining utilities firms – Dwr Cymru Welsh Water, Hafren Dyfrdwy, Severn Trent and United Utilities.

It means it has gathered evidence and found the firms may have breached their obligations to protect the environment, which could lead to fines.

Ofwat opened a major investigation in November 2021 into the companies’ sewage treatment works.

It has handed fines amounting to more than £300 million in recent years to the companies it found to have breached their legal obligations.

The regulator has already taken out enforcement cases against Anglian Water, Northumbrian Water, South West Water, Thames Water, Wessex Water and Yorkshire Water, with extensive investigations ongoing.

It is also still monitoring Southern Water following a previous enforcement case in 2019.

David Black, Ofwat’s chief executive, said: “The fact that Ofwat now has enforcement cases with all 11 of the wastewater companies in England and Wales demonstrates how concerned we are about the sector’s environmental performance.

“This is the largest and most complex investigation Ofwat has undertaken.”

But Mr Black said the regulator wants to conclude the cases “as quickly as possible”, so that the sector can focus on spending £88 billion as part of its latest plan to “deliver cleaner rivers and seas”.

United Utilities said in a statement: “We understand and share people’s concerns about the health of the environment and the operation of wastewater systems, including combined sewer overflows.”

Severn Trent said it had “committed to going further and faster than anyone else in the sector” in relation to tackling sewage overflows.

Both firms said they would “work constructively” with Ofwat in its investigation.

A spokeswoman for the Government’s Environment Department said: “We will never look the other way while water companies pump sewage into our waterways. That is why we welcome the enforcement action announced today.

“The new Government will go even further to clean up Britain’s rivers, lakes and seas.”

Last week, new Prime Minister Sir Keir Starmer said the Government will need to “get to grips with” sewage and pollution, while Environment Secretary Steve Reed proposed new measures to reform the sector and “fix our broken sewage system”.

It followed Ofwat setting out new plans aimed at delivering a 44% reduction in spills from storm overflows compared to levels in 2021.

Storm overflows are designed to work as relief valves by releasing extra rainwater and wastewater into rivers or seas.