Magnus Carlsen, the reigning world chess champion and the top-ranked player globally, faced an unexpected disqualification from the 2024 World Rapid and Blitz Chess Championships.
The International Chess Federation (FIDE) cited a breach of the tournament's strict dress code as the reason for the decision.
In a statement, FIDE said its dress code regulations were designed to "ensure professionalism and fairness for all participants".
The body added that it had issued Carlsen a $200 fine and given him an opportunity to change into the correct attire, which the world number one rejected.
Carlsen, world champion between 2013 and 2023, said he had a lunch meeting before the round and had to change quickly.
"I put on a shirt, jacket and honestly like I didn't even think about jeans, even changed my shoes," Carlsen told.
"I didn't even think about it... First of all, I got a fine which is fine, and then I got a warning that I would not be paired if I didn't go change my clothes. They said that I could do it after the third round today.
"I said 'I'll change tomorrow if that's OK, I didn't even realize it today', but they said, 'Well you have to change now'. At that point it became a bit of a matter of principle for me."
The 34-year-old added that he would not appeal the decision, saying: "Honestly, I am too old at this point to care too much.
"If this is what they want to do... I guess it goes both ways, right - nobody wants to back down - and this is where we are. It's fine by me. I'll probably head off to somewhere where the weather is a bit nicer than here."