Australian swimmer Mack Horton was warned by FINA after snubbing his Chinese rival Sun Yang on the podium after the 400m final.
Horton refused to step up to the top of the podium to take pictures after the award ceremony in Gwangju, while doping allegations swirled around Sun.
The clearly visible protest sparked their ranks at the 2016 Olympic Games in Rio, where the Australian called Sun a “drug scam” over an earlier doping block before turning it into gold.
“The FINA executive… has decided to send a warning letter to Swimming Australia and Mack Horton,” said a statement from the Swimming Board.
“While FINA respects the principle of freedom of expression, it must be conducted in the right context.
“As with all major sports organisations, our athletes and their environment are aware of their responsibility to comply with FINA regulations and not to use FINA events for personal statements or gestures,” she added.
Sun, who has always maintained his innocence about the three-month suspension he served in 2014, is again under a cloud of doping after a report leaked by the FINA Doping Committee claimed to have smashed blood samples with a hammer after being visited by testers last year.
FINA has cleared Sun for competition, but the World Anti-Doping Agency has appealed the decision to the Lausanne Court of Arbitration for Sports (CAS).
“The matter against which Mack Horton allegedly protested is currently being investigated by CAS and it is therefore not appropriate for FINA to prejudge this hearing with further comments,” the FINA statement states.
Horton, who won silver more than 400m behind Sun, was again able to clash with Sun in the 800m freestyle.
Both men will qualify on Tuesday morning, although not in the same heat.
Swimming Australia expressed its support for Horton on Tuesday in response to the FINA warning.
“Swimming Australia respects Mack Horton’s position at the award ceremony and understands his sense of frustration,” said CEO Leigh Russell.
“We are very proud of the performance of our dolphins in Gwangju so far and want to enable them to concentrate on their races for the rest of the competition”.