WARNING: Explicit content.

Italy's Alberto Nonino suffered a nightmare wardrobe malfunction at the U20s World Championships, where his genitalia fell out of his shorts.

The decathlete was competing in the 400m event at the end of the first day of competition in Cali, Colombia, when disaster struck.

The 18-year-old started well out of the blocks, but he was soon seen repeatedly reaching towards his crotch instead of swinging his arms as he ran down the home straight.

Eagle-eyed viewers spotted the unfortunate reason Nonino's run was compromised, with him trying to contain his thunder as it repeatedly fell out during the race.

Replays showed he started well but began to drift further and further behind as he reacted to his situation.

He finished last with a time of 51.57 seconds.

Journalist David Sanchez de Castro's summation of the incident — and Nonimo's manhood — has since gone viral.

"Perhaps I've explained myself poorly. His penis escaped out of the side of his shorts and he had to hold it because it wasn't allowing him to run properly, which is normal when your dongle is swinging from side-to-side," the reporter said.

Nonino has since taken to Instagram to show his fury about how the incident has been reported around the world.

"I just want to talk to you a little bit about the rumpus there's been on blogs and social media in general," he said.

"I'm conscious it was obviously an accident and I'd like to tell you I'm aware of the reaction and you don't need to send me the links to the blogs out there.

"I'm trying to laugh about it now but immediately afterwards I felt terrible and I'm thankful to my friends and family for helping me get over what happened a few hours later.

"The journalistic world worries about cases of bullying throughout Italy and around the world and then publishes these articles that for a more sensitive person could have done a lot of harm."

Spanish press agencies have speculated that Nonino's episode could have been the result of an absence of underwear.