Black clumps of hardened tar dotted with plastic have been spotted on beaches in Tenerife, with researchers warning the substance is unlike any plastic pollution seen before.
The 'plastitar' – described as an "unassessed threat" – was spotted in water near the Playa Grande.
Javier Hernandez-Borges, an associate professor of analytical chemistry at the University of La Laguna in Tenerife, told The Guardian: "No longer is the presence of plastic in the environment limited to microplastics or a bottle in the sea.
"Now it's giving rise to new formations; in this case, one that combines two contaminants."
Plastitar is formed when oil spills wash ashore as tar balls in the Canaries, and sticks to plastic.
Hernandez-Borges said: "It acts like Play-Doh. And when waves carrying microplastics or any other kind of marine debris crash on to the rocks, this debris sticks to the tar."
The plastitar was widespread, the researchers said, and may be related to the Canary Islands position along a key shipping lane for oil.
Hernandez-Borges added: "We're convinced that this is probably found wherever you see this combination of tar – which unfortunately remains common on beaches – and microplastics."
The mixture of hydrocarbons and microplastics could potentially leak toxic chemicals, he said.
Campaigners warned last year that British beaches are being inundated by a form of plastic pollution that looks exactly like rocks.
Watch: Guatemala beach covered in plastic waste in shocking video
The so-called 'pyroplastics' are believed to be remnants of plastic that has been burnt or melted.
Hilary Rowlands, a founding member of Tywyn Beach Guardians in Gwynedd, told North Wales Live: "It's only when you pick them up, and feel how light they are, that you realise they are not stones at all.
"There's not a single beach I’ve combed where I haven’t come across them. Sometimes they are covered in oil or impregnated with the toxins that come from burning plastic.
"It’s all dangerous, both to the environment and the marine life. The longer-term concern is that they will break down into microplastics and threaten marine food chains."
Pyroplastics look almost exactly like pebbles, and are created when plastics are heated during manufacturing processes.
Researchers began to analyse the ‘rocks’ in recent years after people spotted them on beaches in Cornwall - initially thinking they were real pebbles.
The lumps of plastic also weather like real rocks, and shed microplastic into the environment.
Some of the lumps could be as much as half a century old, it is believed.
Pyroplastics are found worldwide, with samples having been located on Atlantic beaches in Spain and the Pacific beaches of Vancouver.
Watch: The state of our oceans on World Oceans Day