The Environment Agency found “poor maintenance” during its 730 inspections of Southern Water wastewater assets in the past year.
The government regulator found “widespread problems like poorly maintained screens and inadequate management of storm tanks”.
The most common issue was found to be noncompliance around “discharge sample and discharge points”, which were “in inaccessible locations or operators did not know where these points were situated”.
Southern Water has been told to take more than 400 actions to comply with Environment Agency permits and prevent further action.
Of the sites investigated, 68 per cent were found to be compliant with their permits.
Southern Water said its turnaround plan in April 2023 “delivered real change by spring 2025” and the “vast majority of the 400 required actions relate to relatively minor breaches”.
It cited “a range of improvements”, such as reduction in overall pollution by 30 per cent, internal sewer flooding by 40 per cent and leakage by 18.5 per cent.
Its drinking water quality compliance score is also up by 40 per cent, Southern Water said.
Dawn Theaker, water industry regulation manager for the Environment Agency, said: “Environment Agency inspectors are working very hard visiting hundreds of sites, each a key part of the drinking water and sewage systems everyone relies upon.
“These health checks find issues Southern Water needs to fix because if things go wrong, the environment suffers. We will keep returning until faults are fixed, and we’ll keep coming back to make sure everything stays that way.”
Helen Wakeham, Environment Agency’s director for water said: “In our role as regulators of the water industry, we are changing how we operate – with better data, our largest ever enforcement workforce and greater powers to do our job effectively.
“Inspections are a vital preventative measure, with our teams nationally issuing over 3,000 actions to water companies, including repairing sewage works and upgrading their infrastructure.
“Together, this will drive meaningful improvements in performance, hold persistent offenders to account and ultimately create a cleaner water environment.”
A spokesman for Southern Water said: “We continue to be open and transparent, and work closely with the Environment Agency and local stakeholders, including MPs, to meet the standards our customers and communities expect.
“That includes taking action whenever issues are identified.
“Pollution incidents have fallen by 30 per cent since 2023, reflecting the real progress we’re making on environmental performance.
“We’re investing more than £8.5 billion between 2025 and 2030 to upgrade infrastructure for customers and to protect the environment.”
