Untreated sewage was released into designated shellfish waters for 192,000 hours last year, new research has found.
The dirty water pouring into English seas was a 20% jump from 159,000 hours in 2022, according to the analysis of Environment Agency data by the Liberal Democrats, shared with Sky News.
The number of individual incidents in 2023 reached 23,000, the equivalent of 64 times a day, a slight fall from 24,000 in 2022.
Some fishing waters in Cornwall were forced to close last year after high levels of e.coli were found in oysters and mussels, and norovirus can also be transported via human waste.
While the fishing industry can use sterile seawater to clean their catch before it reaches the plate, it has branded the situation a "stitch-up" because it foots the bill for the process.
Liberal Democrat environment spokesperson Tim Farron MP said: "This environmental scandal is putting wildlife at risk of unimaginable levels of pollution.
"The food we eat, and the British fisheries industry, must be protected from raw sewage."
The Lib Dems are calling for an investigation into shellfish water quality - which should be protected from deterioration under the Water Framework directive - and a government clampdown on polluting companies.
"It is getting worse on their watch and there will be real concerns for the fishing industry if this trend continues," added Mr Farron, whose party is targeting many rural seats in the upcoming general election.
The worst offender was South West Water, responsible for 13,000 sewage discharges, totalling 98,000 hours, followed by Southern Water, which released sewage 7,000 times for 73,000 hours.
Southern Water pointed to the fact 2023 fell in the wettest 18-month period on record, while South West Water said it has a high proportion of shellfish waters across its vast West Country coastline.
Just 9% of shellfish waters are classified "class A" - clean enough that shellfish harvested from them can be sold without purifying first.
Anything harvested from lower quality water must be cleaned first in depuration tanks, where the molluscs purge themselves with sterile water, or cannot be sold at all.
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Fishing industry on a 'knife edge'
Martin Laity, of Sailors Creek Shellfish, has been catching native oysters from the waters of Cornwall for 34 years.
He tracks alerts on the latest sewage discharges, so he can avoid fishing in those waters, and sometimes soaks the oysters in purification tanks for days longer than mandated just to be safe.
He calls the situation a "stitch-up" because it pushes up producers' electricity and labour costs, and reduces the value of their catch, for which they receive no compensation.
Joe Redfern from the Shellfish Association Of Great Britain said producers "live on a knife edge".
"Just one bad result can shut down their business overnight, leading to huge impacts to their business. It is a desperate situation and one that seems to be getting worse, with some businesses shutting for good," he said.
It wants compensation for producers from the fines the government imposed on water companies for excessive sewage releases.
A spokesperson for industry body Water UK said: "Water companies understand and sympathise with the issues these businesses and coastal communities are facing, which is why we are proposing to spend £11bn to reduce spills as quickly as possible, halving spills into shellfish water by 2030."
An environment department (Defra) spokesperson said: "We're already taking action to clean up shellfish sites by driving the water industry to deliver the largest infrastructure programme in history - £60bn over 25 years - to cut spills by hundreds of thousands each year.
"Shellfish sites will be prioritised alongside bathing waters and sites of ecological importance."
Defra is also increasing inspections and regulator funding, and considering banning some water company bonuses, they added.
South West Water said its plans will ensure all shellfish sites in its area meet the government's target of less than 10 spills per year by 2030, and Southern Water said shellfish can also be infected by farming, run off from roads, boats, marine life and pesticides.
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