Plastic particles and man-made fibres have been found in the guts of more than 100 sea turtles from around the world.
Scientists carried out tests on turtles that had died after becoming stranded or caught in fishing nets.
The animals spanned three oceans, the Atlantic, Pacific and Mediterranean, and included members of every known sea turtle species.
The researchers looked for synthetic particles less than 5mm in length and found them in every one of the 102 turtles studied.
Fibres from sources such as clothing, tyres, cigarette filters, ropes and fishing nets were the most common types of particle discovered.
In total, 800 individual particles were recovered.
But since only part of each animal's gut was studied, the true number ingested could be 20 times higher, the researchers said.
Likely sources of the particles are thought to be polluted seawater and sediments, or consumption of prey or plants.
"At the moment, this is not the main threat to this species group but it is a clear sign that we need to act to better govern global waste," Senior author Professor Brendan Godley, also from the University of Exeter, said.
The worst affected turtles were from the Mediterranean, the study found.
The Exeter team worked in collaboration with Greenpeace scientists.
The findings are published in the journal Global Change Biology.
