Welsh Water has admitted illegally spilling untreated sewage at dozens of treatment plants for years.
The admission came after the BBC presented the water company with analysis of its own data.
One of their worst performing plants is in Cardigan in west Wales.
The company has been spilling untreated sewage there into an environmentally protected area near a rare dolphin habitat for at least a decade.
Welsh Water says it is working to tackle the problems and does not dispute the analysis, which was shared with BBC News by mathematician and former University College London professor Peter Hammond from campaign group Windrush Against Sewage Pollution (WASP).
Most of the UK has a combined sewerage system, meaning that both rainwater and wastewater - from toilets, bathrooms and kitchens - are carried in the same pipes. Usually, all the waste is carried to a sewage treatment works.
During heavy rain, to prevent a plant becoming overwhelmed, it is allowed to discharge untreated sewage. But releasing any before a plant reaches the overflow level stipulated on its permit is an illegal breach.
Prof Hammond requested data on 11 Welsh treatment plants and found that 10 had been releasing untreated sewage at times when they should have been treating it.
Cardigan was particularly bad, spilling for more than 200 days each year from 2019-2022.
The data provided to Prof Hammond showed that Cardigan almost never treated the amount of sewage it was supposed to.
According to its permit it has to treat 88 litres a second before spilling - but had illegally spilled untreated sewage for a cumulative total of 1,146 days from the start of 2018 to the end of May 2023.
"This is the worst sewage works I've come across in terms of illegal discharges," he said.