It comes after Welsh Water admitted illegally spilling untreated sewage.

Natural Resources Wales - the body with responsibility for ensuring the country's natural resources are sustainably maintained - said it was looking at the severity and frequency of the company's permit breaches.

Welsh Water were shown data by an environmental campaigner and said between 40 and 50 of its plants where wastewater is treated are operating in breach of their permits.

They have committed to investing to ensure the plant operates within its permit.

Analysis was undertaken by Professor Peter Hammond from the campaign group Windrush Against Sewage Pollution.

It showed the worst offender is the Cardigan plant in West Wales.

It had untreated sewage spilled on 1,146 days between the start of 2018 and May 2023.

The wastewater from the plant is discharged into the River Teifi, which flows into Cardigan Bay.

Cardigan Bay is home to Europe's largest population of bottlenose dolphins.

Water treatment plants are allowed to discharge untreated sewage to avoid becoming overwhelmed during heavy rain.

But the overflow level is regulated by permit.

Welsh Water says it plans to build a new £20m water treatment works for Cardigan to ensure it fully complies with the discharge permit.

Ann Weedy, operations manager at Natural Resources Wales (NRW), said it was aware of "significant compliance issues" at the Cardigan plant.

"The issue is now unresolvable without significant investment and upgrading of infrastructure by Welsh Water. But this is the kind of investment that we and the public expect," she said.

"We are currently investigating the severity and frequencies of the permit breaches at the site and this investigation is ongoing."

A spokesperson for Welsh Water said the firm had a "longstanding record of being open and transparent" on its environmental performance.

"We have applied this approach to compliance with our treatment works permits and have always raised any areas of non-compliance with our regulator, Natural Resources Wales, as soon as they are identified," they added.

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"We have extensive monitoring arrangements, amongst the most extensive in the sector, which we use to monitor our sites.

"Each site presents its own and sometimes very complex challenges, with NRW reviewing the information we provide, agreeing appropriate timescales for resolving the issues, and taking enforcement action where appropriate.

"We always aim to resolve any issues as quickly as possible, to limit any impact for both the environment and our customers."