Sponge-like substances have washed up on beaches in northern France Sea-Mer Association

Beaches in northern France have been overrun by yellow, sponge-like balls – and nobody knows where they have come from.

The spongy substances have collected along a 30km stretch of coastline of France's northern Pas-de-Calais department, according to The Local.

Beaches affected by the strange substances include La Slack, Wimereux, Le Portel, Equihen-Plage, Hardelot, Le Touquet, Stella and Berck, all of which are popular with tourists.

The French authorities aren't sure what the balls are, but researchers are investigating to find out.

Jonathan Hénicart, president of the coastal-protection NGO Sea-mer association, told French news channel BFMTV that he believed the spongy balls were from an oil product.

"It could come from a polyurethane product commonly used for building. And it smells very, very lightly of paraffin," he said.

In January, dog walkers were warned to stop their pets eating a white substance that washed up on beaches in Pembrokeshire, Wales.

The waxy substance, believed to be palm oil, isn't dangerous to humans – but it is toxic to dogs if ingested.