West Bay’s swimming beach had to be closed on Tuesday after a smack of Portuguese Men o’ War were spotted off the coast.

West Dorset coastguards have been giving warnings after an invasion of more than 100 were found up along the coast.

Mike Grocott, RNLI deputy divisional manager for Dorset said: “We understand that there have been a number of sightings of Portuguese Man o’ War along the Dorset coast.

“In particular lifeguards closed the bathing area at West Bay on Tuesday as a precaution after 21 were spotted.”

After a lifeguard’s morning shoreline patrol found nothing on Wednesday all RNLI lifeguarded beaches were re-opened.

“If members of the public do see a Portuguese Man O’War, they should not touch it.

“If you are stung, please seek assistance from the RNLI lifeguards on duty. Treatment involves removing the tentacles by hand with gloves or by spraying with seawater, and immersion in or application of hot water to relieve the symptoms.”

Coastguards have alerted all local authorities and beach owners from Branscombe to Portland to keep an eye out for the potentially deadly creatures and to warn bathers.

A coastguard spokesman said: “We have asked for beaches to be checked to make sure they are clear before anybody goes on them, “If the weather gets better, as is promised, then we want to have the beaches fairly clear. They are not usually lethal but they do have a very nasty sting even when they are dead.

“If people find one, don’t go prodding it. They do crop up but this time we do seem to have a glut of them.

“There were 100 picked up at Branscombe by the coastguard on Tuesday and some at Seaton and Beer.”

One of the treatments for stings is vinegar and lifeguards and coastguards will be equipped to deal with jellyfish victims, he added.

West Bay coastguard chief Rob Malpas said: “In my 35-year career we have not experienced such vast numbers and obviously the higher the numbers the more possibility is of people getting caught by them and also they don’t die when they are washed ashore either so don’t touch them.

“A Portuguese Man o’ War is not just one fish or animal it is several living in like a colony together – very peculiar things that actually float above the water.”

The jellyfish are thought to have been blown towards the coast by southerly and south westerly winds.

Warning signs have been posted on beaches to make holidaymakers aware that more jellyfish could come ashore and giving advice about what to do if stung.

WHAT TO DO IF YOU GET STUNG People stung must get out of the water to avoid drowning.

There are three goals of first aid for uncomplicated stings: prevent injury to rescuers, inactivate the nematocysts, and remove any tentacles.

For eyes, vinegar may be placed on a towel and dabbed around the eyes.

Salt water may also be used as a substitute but don’t use fresh water as it can cause the release of extra venom.

Avoid rubbing the wound, or using alcohol, spirits or ammonia. The stinging cells cannot be removed by removing tentacles.

After large pieces are removed, shaving cream may be applied and a knife edge, safety razor, or credit card used to take away any remaining nematocysts.

To remove the venom in the skin, apply a paste of baking soda and water and apply a cloth covering on the sting. If possible, reapply paste every 15–20 minutes. Ice can be applied to stop the spread of venom until either of these is available.

Detached tentacles and dead specimens (including those which wash up on shore) can sting just as painfully as the live creature in the water, and may remain potent for hours or even days after the death of the creature or the detachment of the tentacle.

Stings usually cause severe pain to humans, leaving whip-like, red welts on the skin which normally last about 2–3 days. The pain should subside after about one hour.

Seek immediate medical treatment if the person stung has difficulty breathing, difficulty swallowing, chest pain, or intense pain at the site of the sting.