Researchers have found tiny pieces of plastic in all the samples of mussels they tested in British seawaters and bought from local supermarkets.
The scientists, from the University of Hull and Brunel University, said the contamination from microplastics and other human debris, such as cotton and rayon, was significant and widespread.
Mussels feed by filtering seawater.
They team says more work is needed to understand the health implications of consuming the seafood.
The group's study is published in the journal Environmental Pollution.
It sampled wild mussels from eight coastal locations and mussels purchased from eight unnamed supermarkets. Some of the shop-bought mussels were imported from abroad.
The investigation found that for every 100g of mussels being eaten, an estimated 70 pieces of tiny debris were also being taken in.
The analysis determined that more debris was present in wild mussels than farmed ones. And in the mussels bought in supermarkets, cooked or frozen varieties contained more particles than those that had been freshly caught.
This is just one more study that confirms the ubiquity of microplastics in the environment. These particles are in many food products, in bottled drinking water; they are even in the air.
"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 Prof Jeanette Rotchell from Hull University.
"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.
"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."
Our environment analyst Roger Harrabin said: "Knowing that you might be eating tiny strands of someone else's underwear will doubtless put off many people from a dish of Moules Mariniere.
"But the very durability that makes plastic fibres so persistent in the environment is the same factor that makes us pass plastic through our bodies without absorbing it.
"That's what makes the researchers so confident that it's pretty unlikely eating mussels will do us any harm.
"If you are really worried about this sort of thing, you could go vegan organic, but you would still be breathing in plastic particles through the air and drinking it in water."