Their sting is not usually fatal but it could be for someone particularly allergic or with an acute medical condition, and bathers have been warned to be on their guard.

The alert came after beaches in Spain and around the Balearics were closed this week because of jellyfish. On the Costa del Sol, Red Cross workers treated 1,400 holidaymakers for stings.

The jellyfish on the Continent are mauve stingers or sea mettles, Pelagia noctiluca, which give a nasty sting but are not generally fatal. Marine experts expect them to reach our shores as global warming brings higher sea temperatures.

The immediate threat to British bathers are lion’s mane jellyfish, which are about 4ft in diameter with hundreds of tentacles up to 20ft long. They have been seen from Caithness to Orkney in the north of Scotland.

People on the east coast should also keep a look out because in previous years they have been spotted along the Northumbrian coast and they could travel south to East Anglia. The jellyfish are also being spotted in greater numbers in Ireland.

Peter Richardson, an expert in the species at the Marine Conservation Society, said that he had seen “clouds and clouds” of the lion’s mane jellyfish off Scotland. They were also in the Isle of Man and Cumbria.

“We don’t think their sting is fatal but it depends if people are particularly sensitive,” he said. “I have known of people stung by a lion’s mane who had a fever for 24 hours.

“There are also lots of moon jellyfish in the area and one theory is that the lion’s mane are following the moon type and are feeding on them.” Moon jellyfish, Aurelia aurita, are common throughout British waters and have a mild sting.

Dr Cathy Lucas, a marine biologist at the National Oceanography Centre in Southampton, said: “You get blooms of lion’s mane jellyfish when there are warm, still, conditions which allow them to aggregate. Prevailing winds and sea currents then push them towards the British coastline from the Atlantic.”

The invasion of mauve stingers on Spanish beaches is also related to warm, dry winters and springs, which reduce the freshwater river flow into the sea. Dr Lucas said: “If you don’t get freshwater flow it allows these jellyfish to get close to the shore.”

The Portuguese man-of-war, Physalia physalia — actually a form of plankton and not a jellyfish — can give a fatal sting. They have been seen around Britain, though usually in their juvenile form, known as bluebottles, whose sting is milder.

• Fifty pupils at a surfing school in Cornwall were ordered out of the sea after a potentially deadly Mako shark was sighted off Gwithian beach, near Hayle.

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