Scientists have warned the creatures, which are not strictly jellyfish but floating colonies of microscopic hydrozoans, could soon arrive in waters around the Balearic Islands and the Catalan coast.

With a sting 10 times stronger than an ordinary jellyfish, they present a more dangerous threat than the annual jellyfish invasion of beaches in Spain, France, Italy and North Africa.

Stings from the Man O' War leave whip-like, red welts on the skin which normally last about two to three days. After the initial sting, the pain should subside after about one hour.

Very rarely a sting can be fatal, particularly if the tentacles wrap around a swimmer's neck and cause an allergic reaction, affecting the lungs and airways.

Treatment for Man O' War stings elsewhere on the body involve washing the affected area with salt water and then applying ice to dull the pain.

Colonies of Portuguese Man O' War, resembling a sea of blue plastic bags, have been sighted off the Andalusian coastline near Almeria and along the Costa del Sol between Cadiz and Malaga.

"Climate change is changing the migration patterns of many creatures. If they establish themselves it would be very worrying because they really are very dangerous," Xavier Pastor, the European director of the Oceana ecological campaigning group, told the Independent.

Even dead or washed up on shore the creatures still pose a threat because their tentacles retain their poison.

"The Portuguese Man O' War hasn't been seen in the Mediterranean for a decade, and its appearance off the Spanish coast could herald a process of colonisation, which has happened with other invading species," Mr Pastor said.