Heat brings out jellyfish plague: Bathers warned after large numbers of the fish are spotted near beach hotspots Worst affected parts of the country are beaches in Devon, Cornwall and north-west coast Only one sighting of the deadly Portuguese Man of War in Cornwall Thousands of blue jellyfish are floating near the south-west shore With the school holidays underway, the merest hint of sunshine could see millions of families flocking to the coast. But beachgoers should exercise caution, the hottest July in six years has a sting in its tail - an "explosion" in jellyfish sightings. The tentacled creatures thrive in warmer waters, and global numbers having been rising steadily, with large fluthers- collective noun for jellyfish- now gathering around our shores. Worst affected parts of the country include beaches in Devon and Cornwall, and the north-west coast, according to the Marine Conservation Society (MCS). Beware: Swarms of jellyfish have arrived in our waters due to the recent heatwave and are gathering around our shores Sightings include Lion's Mane jellyfish, which possesses one of the most painful stings in British waters. The species, which can grow several feet wide and is the largest in the world, gets its name from the mass of vivid yellow-orange tentacles trailing down from its body. They have been spotted frequently between the north Wales and Blackpool coast. Experts warn people to "look but not touch" if they should come across them on the beach, and stay out of the water altogether. Those unfortunate enough to be stung can be left with painful red welts and blisters, and can even trigger a heart attack. Holidaymakers are also being told to watch out for blue jellyfish, with numbers "absolutely exploding" across south-west in recent weeks, as thousands float near the shore. It has a mild sting, not dissimilar to that of a nettle, but can trigger anaphylactic shock in those who are allergic. 'Look but don't touch': Swimmers and sunbathers are advised to keep their distance from jellyfish and to stay out of the water altogether So far this year, only one deadly Portuguese Man of War has been sighted, in Cornwall. The MCS's biodiversity programme manager Dr Peter Richardson said: "The scarcity of jellyfish reports before June was unusual and could well be linked to the exceptionally cold spring. However, as our waters warmed, sightings of jellyfish increased drastically. Certainly on the north-west coast - Liverpool bay up to Blackpool - and the north coast of Wales, there have a raft of sightings Lion's Mane jelly fish. For visitors to the British seaside, apart from the Portuguese Man of War, it is the one that has the most powerful sting. Rare: There has only been one sighting of the deadly Portuguese man-of-war jellyfish in Cornwall They can be quite nasty and are definitely ones to avoid, although we are not aware of any fatalities. But some people report it is like being struck by an electric shock, while others can go into anaphylactic shock. There has also been plentiful reports of compass and blue jellyfish in the South West. Although their sting is mild, families should still be on the lookout. Dr Melanie Austen is Head of Science at Plymouth Marine Laboratory, said: "There is definitely greater awareness of jellyfish numbers and probably an increase in recent years, although it is hard to make a comparison without historical records. We think the rise is a result of warming waters due to climate change, which means there is more food around for the jellyfish. And overfishing may have also played a role, with a lack of larger fish that eat them and help keep numbers down. The MCS is urging people to report their sightings of jellyfish by filling a form on their website, as the species act as a barometer of the seas. The UK survey follows a mass invasion of thousands of miles of the Mediterranean coastline by millions of jellyfish in June, to the annoyance of sun-seeking tourists who have flown there this summer.