Particles of debris from car tyres are ending up in the ocean as "plastic soup", conservationists warn.
Microplastics from tyres and textiles are a bigger source of marine pollution than the breakdown of larger plastic waste in some areas, says the IUCN.
Up to 30 per cent of plastic released into the oceans each year comes from primary microplastics, not the disintegration of larger pieces, a report found.
Debris from tyre abrasion and synthetic fabrics are the main sources, they say.
The IUCN reviewed data from seven global regions to look at how much of the estimated 9.5 million tonnes of new plastic waste released into the oceans each year comes from primary microplastics.
These are tiny plastic particles from the likes of consumer products rather than the degradation of larger bits of plastic in the oceans.
The report found between 15 per cent and 31 per cent of plastic pollution came from primary microplastics, of which the biggest contributors (almost two-thirds) were abrasion of synthetic textiles, while washing, and abrasion of tyres, while driving.
Synthetic rubber, made from a variant of plastic, makes up around 60 per cent of the rubber used in tyres.
Other sources included microbeads in cosmetics, which contributed about 2 per cent of the releases to the ocean globally.
François Simard, deputy director of IUCN's marine programme, said the findings came as a surprise.
"We discovered that most of the microplastics are coming from either the clothes or from the tyres," he told BBC News. "Microplastics are going everywhere in the sea and into the food chain, let's close the plastic tap.