The Daily Star's FREE newsletter is spectacular! Sign up today for the best stories straight to your inbox Sign up today! Thank you for subscribing See our privacy notice Invalid Email

(Image: GETTY/BARCROFT)

Experts have long warned athletes about the city’s filthy water as being a big threat to the world’s sailors.

But the potentially-lethal Portuguese man o’ war could pose a far greater danger – with swimmers due to dive into the water around the Copacabana in under a fortnight.

Photographer Daniel Botelho caught the purple jellyfish floating in the shallow waters off the coast of Rio with Sugarloaf mountain clearly visible in the background.

(Image: BARCROFT) (Image: GETTY)

He said: "I was in the water with my brother and his wife when suddenly she started to scream: Something is biting me. Help me please!

"I took one photo, two photos and bang! It felt like hot iron on my skin.

"But I kept shooting until as many tentacles as I could stand got me. After making sure I got a few images, I decided to leave the water.

"It took about 12 hours for the pain go away, and some fever happened, and then an unpleasant itching."

(Image: REDDIT)

The jellyfishes' tentacle can stretch up to 150ft in length and their sting is so painful to humans it can even kill.

And Olympic swimmers could now face a daunting gauntlet of tentacles when they swim in the 10km event.

The Atlantic Portuguese man-o'-war is also known as the blue bottle or floating terror.

It is a marine hydrozoan found in the Atlantic, as well as the Indian and Pacific Oceans.