A swimmer at the Olympics was disqualified over a little-known rule and collapsed to the floor as he found out.

OlympicsCredit: Christian Liewig – Corbis / Getty

Most Olympians spend the majority of their lives training for their event, so they need to make sure they’re mentally and physically ready when the times comes.

So when thousands of hours of training is quickly undone by breaking rules that many aren’t aware of, it can certainly sting.

This is exactly what happened to British swimmer Luke Greenbank, who was a top contender in the 200m backstroke.

Greenbank dominated his heat on Wednesday night, emerging as the fastest qualifier for the semi-finals.

His confident exit from the pool quickly turned to heartbreak when the stadium’s big screen flashed the news of his disqualification following a video review, as reported by the Daily Mail.

GettyImages-2164594163.jpgGreenbank was disqualifed from the mens 200m backstroke. Credit: Maddie Meyer/Getty

The 26-year-old was disqualified for exceeding the underwater 15-meter mark at his third turn.

Replays confirmed that Greenbank had indeed remained underwater past the red marker, leaving no grounds for an appeal.

Upon seeing the result, Greenbank was visibly shocked. He placed his hands on his head and then collapsed to the pool deck as he realized his Olympic dream was over.

“I don’t know what to say, absolutely gutted,” Greenbank said after the disqualification. “It’s really annoying, I feel like I’m on good form.”

The disqualification denied Britain a potential third gold medal of the day, following stunning successes by Adam Yee in the men’s triathlon and the women’s quadruple sculls rowing team.

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Luke Greenbank. Credit: Insidefoto/Getty

The full rule states: “Some part of the swimmer must break the surface of the water throughout the race.

“It is permissible for the swimmer to be completely submerged during the turn, at the finish, and for a distance of not more than 15 meters after the start and each turn. By that point, the head must have broken the surface.”

The 15-meter rule is applied in backstroke, butterfly, and freestyle events but not in breaststroke.