Australian swimmer Mack Horton has been warned by FINA after he snubbed his Chinese rival Sun Yang on the podium after the world 400 metres freestyle final.
Horton refused to step onto the top step of the podium for photos after the medal ceremony in Gwangju, as doping allegations swirl around Sun.
The highly visible protest reignited their row at the 2016 Rio Olympics, where the Aussie called Sun a "drug cheat" over a previous doping ban before pipping him to gold.
"The FINA executive... has decided to send a warning letter to Swimming Australia and to Mack Horton," said a statement from swimming's governing body.
"While FINA respects the principle of freedom of speech, it has to be conducted in the right context.
"As in all major sports organisations, our athletes and their entourages are aware of their responsibilities to respect FINA regulations and not use FINA events to make personal statements or gestures," it added.
Sun, who has always protested his innocence over the three-month suspension he served in 2014, is again under a doping cloud after a leaked FINA doping panel report alleged he smashed blood samples with a hammer after being visited by testers last year.
FINA cleared Sun to compete but the World Anti-Doping Agency has appealed the decision at the Lausanne-based Court of Arbitration for Sport (CAS).
"The matter over which Mack Horton was allegedly protesting is currently under review by CAS and therefore it is not appropriate for FINA to prejudice this hearing by commenting further," the FINA statement said.
Horton, who took silver behind Sun in the 400m, could clash with Sun again in the 800m freestyle.
Both men go in Tuesday morning's qualifiers, although not in the same heat.
Swimming Australia voiced its support of Horton on Tuesday in reaction to the FINA warning.
"Swimming Australia respects the position Mack Horton took during the medal ceremony and understands his sense of frustration," said CEO Leigh Russell.
"We are very proud of our Dolphins' performances in Gwangju so far and want to allow them to focus on their racing for the remainder of the meet."