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Water industry regulator Ofwat has expanded its investigation into sewage spills to include Dwr Cymru Welsh Water and three others.

This means that all 11 water companies covering England and Wales are now subject to enforcement.

In addition to Dwr Cymru Welsh Water, which is headquartered in Nelson, formal notices have also been served to Hafren Dyfrdwy, Severn Trent, and United Utilities to gather evidence for the investigation.

Ofwat says the opening of enforcement comes after analysis on how often companies spill sewage from storm overflows and over “concerns that these companies may not be fulfilling their obligations to protect the environment and minimise pollution”.

The regulator already has enforcement cases against Anglian Water, Northumbrian Water, South West Water, Thames Water, Wessex Water and Yorkshire Water after beginning its investigation in 2021.

Water companies are allowed to release untreated sewage into rivers and seas when it rains to release pressure from the system and prevent flooding in the streets and in people’s homes. This practice is illegal when it is dry weather though.

BBC Wales reported in October last year that Welsh Water, which is a not-for-profit company, had been spilling sewage in the dry.

David Black, Ofwat chief executive, said enforcement cases against all 11 companies shows how concerned it is.

He said: “Where we find that companies have breached their obligations, we will continue to act. Over recent years, we have imposed penalties and payments of over £300 million on water and wastewater companies. 

“This is the largest and most complex investigation Ofwat has undertaken. However, Ofwat is committed to concluding these cases as quickly as possible.”

A Dwr Cymru Welsh Water spokesperson said: “The environment is at the heart of everything we do. Only last week Ofwat outlined its initial assessment of our business plan for 2025 to 2030 which could see over £2.8bn expenditure on services to improve our wastewater performance.

“We will not shy away from the challenges facing us and to comply fully with Ofwat’s investigation and be open and transparent with the information we share with them.

“We have been working closely with our regulators on the monitoring of our wastewater treatment works and are investing at record levels to improve service. We will continue to work with all our regulators to ensure environmental performance improves.”