Australian swimmer Mack Horton has publicly reignited his feud with Chinese rival Sun Yang, years after accusing him of being a "drug cheat".
Sun pipped Horton to claim gold in the 400m freestyle at the World Aquatics Championships in South Korea on Sunday.
Horton later refused to share the medal podium with Sun - who has faced fresh claims of violating doping protocols.
Afterwards, Sun told reporters: "Disrespecting me was OK, but disrespecting China was unfortunate."
He added: "I feel sorry about that."
Horton, the defending Olympic champion, had also declined to shake hands with Sun or pose for photos with him. He happily posed beside Italian bronze medallist Gabriele Detti.
Sun served a three-month suspension in 2014 for testing positive for a banned stimulant trimetazidine, which he said had been for a heart complaint.
The pair's enmity goes back to the 2016 Rio Olympics when Horton accused Sun of deliberately splashing him in a training session, saying: "I ignored him, I don't have time or respect for drug cheats."
Later, he added: "I just have a problem with athletes who have tested positive and are still competing."
Horton won gold in Rio, but Sun has otherwise dominated the event in recent years - his win on Sunday in Gwangju is his fourth consecutive world title.
After claiming silver, Horton was asked how he felt and replied: "Frustration is probably it. I think you know in what respect."
"I don't think I need to say anything. His actions and how it has been handled speaks louder than anything I could say."
Sun said he was "aware of the rumours" but did not elaborate, saying "I will keep trying to put a lot of effort in my swimming".
His comment about "disrespecting" China follows much antipathy from his fans towards Horton in 2016, when the Australian's social media accounts were flooded with abuse.
Other swimming figures have expressed support for Horton's actions in Gwangju.
"Absolutely awesome to see @_mackhorton protesting clean sport by not getting up on the podium next to Sun Yang," tweeted Horton's ex-teammate David McKeon.
Sun faces a fresh hearing at the Court of Arbitration for Sport in September, after the World Anti-doping Agency appealed against a decision to clear him of a separate doping offence.
Reports have alleged that he avoided a drugs ban after destroying blood samples. Sun has denied the allegations.
Last week, Australia's Daily Telegraph posted a leaked 59-page report by the Fina doping panel following a hearing at which Sun said he refused to comply with a drugs test because of doubts over a testers' accreditation.
Chinese swimming officials demanded an apology from Horton after his comments in 2016, but the Australian Olympic Committee (AOC) said he was "entitled to express a point of view".
"He has spoken out in support of clean athletes. This is something he feels strongly about and good luck to him," the AOC said at the time.