All mussels sampled from UK coastlines and supermarkets were found to contain tiny shards of plastic and other debris in a new study.
The scientists behind the report said microplastic consumption by people eating seafood in Britain was likely "common and widespread".
Though they were less certain about the resulting impact on human health, the research team emphasised the importance of further studies to determine any potential harm as a result of people eating plastic.
In samples of wild mussels from eight coastal locations around the UK and eight unnamed supermarkets, 100 per cent were found to contain microplastics or other debris such as cotton and rayon.
Every 100 grams of mussels eaten contains an estimated 70 pieces of debris, according to the researchers, whose study is published in the journal Environmental Pollution.
Mussels feed by filtering seawater through their bodies, meaning they ingest small particles of plastic and other materials as well as their food.
There was more debris in the wild mussels, which were sampled from Edinburgh, Filey, Hastings, Brighton, Plymouth, Cardiff and Wallasey, than in the farmed mussels bought in shops.
But mussels from the supermarkets, which came from various places around the world, had more particles in them if they had been cooked or frozen than if they were freshly caught, the study found.
Analysis shows around half of the debris found in the mussels was microplastics such as polyester and polythene and 37 per cent was other debris including textiles such as cotton and rayon which is made from cellulose.
Seafood is not the only way plastic could turn up in people's diets, as microplastics have been found in other food sources and drinking water, and can even be inhaled, according to Professor Jeanette Rotchell of the University of Hull.
"It is becoming increasingly evident that global contamination of the marine environment by microplastic is impacting wildlife and its entry into the food chain is providing a pathway for the waste that we dispose of to be returned to us through our diet," said Professor Rotchell, who was involved in the study.
"This study provides further evidence of this route of exposure and we now need to understand the possible implications of digesting these very small levels.
Though researchers have expressed concern that plastic accumulating in food chains could harm people and animals, it is currently unclear what this accumulation means for our health.
The most harm currently attributed to plastic ingestion is seen in animals like whales that ingest larger plastic pieces, causing them to choke or starve.
"Chances are that these have no implications, but none the less, there is not enough data out there to say there is no risk. We still need to do the studies and show that is the case,” said Professor Rotchell.
"There are currently regulation of some contaminants in food, in the long term, regulatory solutions to this problem will also be needed."
And, given the proportion of debris such as textiles found in the mussels, she said: "All the conversation is about microplastics, but textiles could also be worth investigation."
Dr Alan Reynolds of Brunel University London, said: "Blue Planet has rightly awoken the public to the devastating effects waste plastics are having on the marine environment.
"This paper highlights that the problems are close to home in finding that these same polluting microplastics are now coming back to us in the food in our supermarkets."
The new research adds to a growing body of evidence that British food is contaminated with plastic.
Previous studies have found that a third of all fish caught in UK waters, including favourites such as cod, haddock and mackerel contain tiny shards of plastic.
Another study suggested that “top European shellfish consumers” might consume up to 11,000 microplastic pieces a year.
Additional reporting by PA
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