Olympian swimmer Neil Agius has described how the sheer number of jellyfish he encountered during an attempt to smash the world record for longest open water swim left him unable to continue.

"There were just too many jellyfish for too many hours," he said in a video posted to social media on Tuesday.

"I'm full of stings everywhere, I didn't feel like it made sense to continue," said Agius. 

Agius was forced to abandon his attempt to cross 160 kilometres of open water shortly before 4am on Tuesday morning after being stung repeatedly while swimming through a swarm of jellyfish. 

The swimmer, who represented Malta at the 2004 Olympics in Athens, was attempting to cross from Mallorca to the west coast of Ibiza. 

Up until that point, the challenge appeared to be going well, with the swimmer reportedly ahead of schedule. 

The athlete encountered the swarm 46km into the swim - more than one-quarter the way through the challenge. 

After attempting to battle through for a further four hours, Agius made the decision to call off the attempt.

"I felt like I could do this forever, but, unfortunately, it wasn't meant to be this time," said Agius, before going on to thank his team for their time. 

"Nature decided otherwise from us continuing on this journey," he said. 

'Relentless' swarms

A scene from the harrowing experience can be seen in the video, which shows Agius swimming cautiously but determinedly as one team member shouts, "There's one in front of you, Neil!"

The swimmer estimates he was stung around 60 times over the course of the ordeal, which he described as "relentless."

He expressed regret at having to abandon the attempt due to the stings, as opposed to being forced to stop due to exhaustion or insufficient nutrition. 

Agius' fiancé Lara Vella appeared to be shocked by the number of jellyfish her partner encountered, seen in the video repeating, "They just wouldn't stop."

She describes how each time Agius was stung, the offending jellyfish would briefly light up a shade of blue.  

In a statement posted to social media shortly afterwards, the swimmer reaffirmed his commitment to raising awareness of ocean-related issues and promised to return. 

"The real challenge for ocean conservation still stands, I will be back," he said.