A study of human stools has found evidence of microscopic plastic particles swallowed in food.
In every sample investigated, scientists discovered tiny particles of up to nine different types of plastic.
Plastic in the gut could suppress the immune system and aid transmission of toxins and harmful bugs or viruses, experts believe.
"Of particular concern is what this means to us, and especially patients with gastrointestinal diseases,” said lead researcher Dr Philipp Schwabi, from the Medical University of Vienna in Austria.
"While the highest plastic concentrations in animal studies have been found in the gut, the smallest microplastic particles are capable of entering the bloodstream, lymphatic system and may even reach the liver.
"Now that we have first evidence for microplastics inside humans, we need further research to understand what this means for human health."
The pilot study recruited eight participants from the UK, Finland, Italy, the Netherlands, Poland, Russia and Austria.
Each person kept a food diary for a week before having stools samples taken.
The diaries showed that every participant was exposed to plastic by consuming plastic-wrapped food or drinking from plastic bottles. None were vegetarians, and six ate sea fish.
Particles between 50 and 500 micrometres across up to nine different plastics were found, the most common being polypropylene and polyethylene terephthalate.
On average, the scientists found 20 microplastic particles in every 10 grams of stool.
It is estimated up to 5 per cent of all plastics produced end up in the sea.
Once in the oceans, plastics are consumed by sea animals and move up the food chain. Significant amounts of plastic have been detected in tuna, lobster and shrimp.
Food is also likely to be contaminated with plastic as a result of processing or packaging, say the researchers whose findings were presented at UEG Week, the largest meeting of gastroenterology experts in Europe.