Large quantities of plastic debris are building up in the Mediterranean Sea, say scientists.
A survey found around one thousand tonnes of plastic floating on the surface, mainly fragments of bottles, bags and wrappings.
The Mediterranean Sea's biological richness and economic importance means plastic pollution is particularly hazardous, say Spanish researchers.
Plastic has been found in the stomachs of fish, birds, turtles and whales.
Very tiny pieces of plastic have also been found in oysters and mussels grown on the coasts of northern Europe.
Plastic is accumulating in the Mediterranean Sea at a similar scale to that in oceanic gyres, the rotating ocean currents in the Indian Ocean, North Atlantic, North Pacific, South Atlantic and South Pacific, the study found.
A high abundance of plastic has also been found in other seas, including the Bay of Bengal, South China Sea and Barents Sea in the Arctic Ocean.
Commenting on the study, published in the journal PLOS ONE, Dr David Morritt of Royal Holloway, University of London, said scientists were particularly concerned about very small pieces of plastic (less than 5mm in length), known as microplastics.
The study found more than 80% of plastic items in the Mediterranean Sea fell into this category.
"These very small plastic fragments lend themselves to being swallowed by marine species, potentially releasing chemicals into the gut from the plastics," Dr Morritt, of the School of Biological Sciences, told BBC News.
The Mediterranean Sea represents less than 1% of the global ocean area, but is important in economic and ecological terms.
It contains between 4% and 18% of all marine species, and provides tourism and fishing income for Mediterranean countries.