A study of dolphins, seals and whales washed up on British shores found plastic in the digestive system of every one.
Scientists examined 50 animals from 10 different species that had died from a variety of causes.
All of them had "microplastic" particles, less than five millimetres across, in their stomachs and intestines.
The vast majority of the particles were synthetic fibres that may have been shed by clothes or fishing nets.
Others were fragments of originally larger pieces that could have come from food packaging or plastic bottles.
Lead researcher Sarah Nelms, from the University of Exeter, said: "It's shocking – but not surprising – that every animal had ingested microplastics.
"The number of particles in each animal was relatively low, an average of 5.5 particles per animal, suggesting they eventually pass through the digestive system, or are regurgitated.
"We don't yet know what effects the microplastics, or the chemicals on and in them, might have on marine mammals."
Animals that died as a result of infectious disease had slightly higher numbers of particles than those killed by injury or other causes.
Professor Brendan Godley, from the University of Exeter’s Centre for Ecology and Conservation, said it was not possible to draw any firm conclusions on the significance of the link.
But he added: "We are at the very early stages of understanding this ubiquitous pollutant.
"Marine mammals are ideal sentinels of our impacts on the marine environment, as they are generally long-lived and many feed high up in the food chain.
"Our findings are not good news."
The team, whose findings appear in the journal Scientific Reports, said bacteria, viruses and contaminants carried on the plastic were a cause for concern.
Species studied included the Atlantic white-sided dolphin, the bottlenose dolphin, the common dolphin, the grey seal, the harbour porpoise, the harbour seal, the pygmy sperm whale, Risso’s dolphin, the striped dolphin, and the white-beaked dolphin.