Japan's WBA light flyweight super champion Hiroto Kyoguchi has tested positive for the coronavirus, his gym announced Monday, forcing organisers to scrap his title defence against Thailand's Thanongsak Simsri.
Kyoguchi, 26, who had been due to face Thanongsak in Osaka on Tuesday, tweeted simply: "I'm really sorry" without giving any further details.
"We learned today that Kyoguchi and his trainer tested positive for the coronavirus," a staff member at Kyoguchi's gym told AFP.
"The title match scheduled for tomorrow is cancelled," she said, adding it might be rearranged for a later date.
Kyoguchi has so far shown no symptoms, local media said.
Both Kyoguchi and Thanongsak, 20, are currently 14-0 as professionals. Kyoguchi had been due to defend his title in May, but the bout was postponed because of the pandemic.
The Thai boxer had been self-quarantining in Japan ahead of the bout at an Osaka arena where organisers planned to halve the number of spectators to around 2,000 as part of anti-infection measures.
The abrupt announcement came just hours after the two boxers passed their weigh-ins.
It would have been the first men's world title match held in Japan since the pandemic began.
Kyoguchi's local fans appeared understanding of the cancellation, with one tweeting: "You should not be sorry. We are looking forward to the next match."
Japan is making a tentative return to hosting events with international athletes as it navigates difficult questions over how it can stage next year's virus-postponed Olympics.
Questions arose over a gymnastics event featuring China, Russia, the US and Japan when the International Gymnastics Federation said Kohei Uchimura, the two-time men's all-around Olympic champion, had tested positive.
But the test was later revealed to be a false positive, with Uchimura testing negative and now scheduled to take part in the competition this weekend in Tokyo.
Officials are also trialling a series of sports events with capacity crowds with an eye on next year's Games.
On Friday, thousands of fans flooded into a baseball stadium near Tokyo that will host Olympic events next year as a test for coronavirus prevention measures.