Vast numbers of jellyfish have washed up on beaches across Pembrokeshire in what conservationists have described as a ‘mega swarm’ phenomenon.

The barrel jellyfish can grow up to three feet in diameter and even though they are relatively harmless, beach-goers have been warned not to move the creatures as they can sting.

Commenting on the phenomenon to hit the UK coastline, one marine wildlife expert said that she had never seen jellyfish this big before, and put it down to the recent warm weather and a series of mild winters which had allowed the creatures’ main food source of plankton to surface.

Holidaymaker Katie Bradshaw, who was down from Warrington with her boyfriend celebrating her birthday in Tenby over the weekend, took the picture on the South Beach on Sunday and discovered more washed-up jellyfish a little further along on Castle Beach that same evening.

“We didn’t think Tenby had jellyfish, let alone huge ones!” she said.

Meanwhile, Nicky Mallen, who took the photograph at Saundersfoot, told us: “Barrel Jelly Fish (Rhizostoma pulmo) are gentle giants and, although infamous for their stinging tentacles, most species are harmless to humans. Turtles love them as a tasty snack and for the past couple of years we seem to be getting rather large specimens stranded on Saundersfoot beaches.

“These bizarre creatures have drifted through our oceans for over 500 million years, despite lacking a brain, heart or blood. Their soft bodies are over 90 per cent water, and they move in a mesmerising pulsing motion operated by a simple net of nerves,” she added.

Olly the dog also decided to investigate when he spotted one of the jelly fish while being walked on Tenby’s South Beach by Ken Lloyd, who was ‘dog sitting’ for his daughter Georgina while she was in London with the ‘Heads Together’ team doing a charity walk around he capital.

“They looked like something out of Doctor Who?” said Ken.