A layer of “sea snot” thought to be the biggest on record has hit the Turkish coast, endangering fish and the fishing industry.

A thick slimy film of the organic matter, known as marine mucilage, has spread through the Sea of Marmara south of Istanbul.

Harbours, shorelines and swathes of seawater have been blanketed by the viscous, greyish substance, some of which has also sunk below the waves, suffocating life on the seabed.

Studies show the slime reaches up to 30m deep, threatening to poison crabs and other shellfish.

Although it first happened in 2007, the latest outbreak is believed to be the biggest in history and is causing chaos for local communities.

The slime has also been found in the Aegean Sea in Greece.

“Hopefully, together we will protect our Marmara within the framework of a disaster management plan,” Environment Minister Murat Kurum said.

“We will take all the necessary steps within three years and realise the projects that will save not only the present but also the future together,” Mr Kurum said.

Scientists say climate change and pollution have contributed to the proliferation of the organic matter, which contains a wide variety of microorganisms and can flourish when nutrient-rich sewage flows into seawater.

President Tayyip Erdogan blamed the outbreak on untreated water from cities including Istanbul, home to 16 million people, and vowed to “clear our seas from the mucilage scourge”.