Great Britain had a good day in the pool Wednesday, finishing with a bronze in women’s synchronized 10m diving, but disaster struck during the men’s 200-meter backstroke heats at the 2024 Paris Olympics.

Luke Greenbank finished first in the fourth and final 200 backstroke heat with a time of 1:56, but he didn’t advance to the semifinals. Instead, Greenbank was disqualified from the competition because he was completely underwater past the 15-meter mark following a turn. Greenbank, who won bronze in the event at the Tokyo Olympics in 2021, put his hands on his head in distress and fell to his knees after learning of his disqualification.

“I don’t know what to say, absolutely gutted,” Greenbank said after the race, according to The Sun. “It’s really annoying, I feel like I’m in good form.”

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According to World Aquatics, competitive swimming’s international governing body, swimmers participating in the backstroke can’t be fully submerged more than 15 meters after starting the race or completing a turn. “By that point the head must have broken the surface,” the rules state. There are red buoys along each swimming lane to mark the 15-meter point and although it’s not a common rule known by the public, it is well-known among swimmers.

The final for the 200 backstroke is scheduled Thursday at 2:38 p.m. ET. The field includes Hungary’s Hubert Kós, the 2023 World Aquatics Champion, and Switzerland’s Roman Mityukov, who finished third at the 2023 World Aquatics Championships.

American Ryan Murphy, who won a silver medal at the 2021 Tokyo Olympics and finished second at the 2023 Worlds, didn’t qualify for the final after finishing sixth in his semifinal. Murphy won bronze in the men’s 100-meter backstroke earlier this week.

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This article originally appeared on USA TODAY: Great Britain swimmer disqualified from 200m backstroke. What happened