Holidaymakers heading to the sun, be warned. A plague of jellyfish of “unprecedented” size is gathering in the western Med, marine biologists have warned, forcing resorts from Valencia to Monte Carlo to prepare countermeasures to protect their beaches.

“We detected immense swarms of Pelagia noctiluca, the ‘mauve stinger’, off the Balearic Islands,” says Dacha Atienza, of the Spanish National Research Council (CSIC). “Around Minorca, the jellyfish density reaches 8-15 per square metre.”

When the creatures arrive depends on the wind and tide, but hundreds of thousands of mauve stingers, which have venomous tentacles reaching 10ft in length, have already been washed ashore in the south of France.

Beaches from Cannes to Antibes have been affected, despite the use of nets and a vessel designed to suck the creatures from the water. Rescue services reported more than 500 swimmers stung in a single day on one beach in Nice.

Poison algae and jellyfish empty beaches

Holiday beaches from Rome to Genoa are deserted because of a plague of poisonous algae

Man-of-war threaten South West’s beaches

Beachgoers should be on the alert for Portuguese man-of-war jellyfish, which have been seen this month on the Cornish coast

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The sting can cause asthma attacks, acute allergic reactions and, in rare cases, cardiac arrest.

Dr Andrew Mackay says his holiday was ruined after he swam through a shoal off Minorca last summer. “The pain is excruciating, somewhere between an electric shock and a burn,” he said. “I was only 100 yards offshore but was incapacitated. I was lucky not to drown, and I’m a 6ft man. Imagine what the jellyfish could do to a child.”

Traditional treatments, such as rubbing stings with sand or urinating on them, are ineffective. Toxicologists recommend washing the area with sea water – fresh water only worsens the sting – and removing traces of tentacle with tweezers. Take mild antiinflammatories such as ibuprofen to ease the pain.

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