Gwangju, South Korea: China's Sun Yang and British rival Duncan Scott were rebuked by FINA on Tuesday following a heated exchange after Sun retained the 200m freestyle world title.
Scott, who tied for bronze, refused to shake hands with Sun on the podium in Gwangju after the playing of the national anthem or join him and the other medallists on the top step to pose for photos.
His snub, which follows a similar protest from Australian Mack Horton after losing to Sun in the 400m final at the weekend, sparked a furious reaction from Sun.
The triple Olympic champion shook his fist at Scott before getting in his face, waving his finger at the Briton and barking: "You're a loser -- I win!"
The Chinese giant has faced hostility from swimmers in South Korea after a leaked FINA doping panel report alleged he had used a hammer to destroy vials of his own blood after testers visited him last year.
Swimming's governing body warned Sun and Scott for bringing the sport into disrepute, a FINA statement reading: "Both competitors had an inadequate behaviour on this occasion".
FINA also sent a letter to Horton for a similar podium snub at the weekend, which reignited a bitter feud that goes back to the 2016 Rio Olympics.
Scott, who marched off the pool deck during the post-ceremony lap of honour, was unrepentant.
"If Sun can't respect our sport then why should I respect him?" he said. "I think a lot of people, everyone in swimming, got behind what Mack did."
Horton labelled Sun a "drug cheat" in Rio over a three-month suspension his Chinese rival served for taking a banned stimulant two years earlier.
Sun has always maintained it was medication for a heart condition, while Chinese doping officials are believed to have failed to update their therapeutic use exemptions list for athletes.
The pair's Olympic clash triggered a war of words and almost threatened to spark a diplomatic row as China's state-run media called Australia a "second-class citizen" of the West and accused it of "white supremacy".
The China Daily weighed in on their latest run-in on Tuesday, claiming Horton had "humiliated himself" with his protest.
"Needless to say such moves are rather rude and lack the most basic manners," it added in a commentary.
"Not joining Sun on the podium was not a protest, but an insult."
Meanwhile, Horton has been trolled by fuming Chinese fans on social media since refusing to stand on the podium for the Chinese national anthem, even having death threats made against him and his family.
Several athletes have backed Horton and Scott, including Olympic champions Adam Peaty and Lilly King.
"He's completely right," said Peaty after qualifying fastest for Wednesday's 50m breaststroke final.
"I wouldn't have gone on the podium," he added. "The most important thing as a sportsperson is you have the right to a voice and Duncan showed his voice."
King called Scott's protest "awesome" after slamming FINA's doping controls earlier this week.
And South African Chad le Clos, runner-up to Sun in the 200m Olympic final, said: "I want my gold medal back from Rio. James Guy should get his bronze medal too because it shouldn't be allowed to have people cheating in sports."