Britain’s water industry regulator is now investigating all of England and Wales’ water companies over sewage spills.

Ofwat announced on Tuesday that it has served enforcement notices on Dwr Cymru Welsh Water, Hafren Dyfrdwy, Severn Trent and United Utilities companies.

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.

Southern Water remains subject to enforcement monitoring following a previous enforcement case in 2019.

The action is part of the organisation’s investigation into how companies manage their wastewater treatment works and networks.

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.”

The latest four cases were opened after detailed analysis of data on the companies’ environmental performance and frequency of spills from storm overflows.

Sewage spills from water infrastructure have contributed to a situation in which no single river in England is considered to be in good overall health, and beauty spots including Windermere in the Lake District have been polluted.

Storm overflows – which release untreated wastewater into rivers and seas when there is heavy rain to prevent sewers becoming overwhelmed – dumped sewage into the environment 464,056 times and for more than 3.6 million hours in 2023.

Fines are issued to companies which breach legal obligations to protect the environment. Between the start of 2015 and July 2023, more than £150m were collected through 59 prosecutions for environmental offences.

Mr Black said the regulator wanted 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”.

Funding for the investment will partly come from a rise in household water bills, which has been capped to an average of £94 over a five-year period, instead of the £144 proposed by the water companies.

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.