OSLO – Norwegian world chess champion Magnus Carlsen abruptly withdrew from a game against American Hans Niemann, reigniting an ongoing cheating controversy in the sport.

After playing just one move in their match on Monday night in the Julius Baer Generation Cup online tournament, Carlsen resigned without a word and turned off his webcam in front of stunned commentators.

The 31-year-old five-time world champion did not give an explanation, but his gesture was seen as a further protest against the young American, 12 years his junior.

A week ago, Carlsen withdrew from the Sinquefield Cup in the United States after losing to Niemann, a rising star who has admitted to cheating in online games in the past.

In a message that was widely understood as an innuendo, Carlsen then posted a video from 2014 of football coach Jose Mourinho, then at English team Chelsea, on Twitter.

“I’d prefer really not to speak. If I speak, I am in big trouble,” the Portuguese said in the clip, which many saw as a sign of protest.

Carlsen has not commented further, although the world’s best blitz player, Hikaru Nakamura, claimed then it was the result of Carlsen suspecting Niemann had “probably cheated”.

The Norwegian’s latest withdrawal has reignited the controversy.

“It’s an unprecedented case,” Dutch grandmaster Anish Giri told Norwegian broadcaster TV2.

“I totally understand that from Magnus’ point of view because he doesn’t have full trust in him because he knows about his repeated online cheating. But that said, of course, normally when you have that situation, you kind of deal with it. You just hope your opponent doesn’t cheat and you play.”

Interviewed by website Chess24, American Levon Aronian said he "understands the frustration of Magnus."

"I'm somewhere in the middle. I do believe that Hans has been not the cleanest person when it comes to online chess. But he is a young guy, hopefully this will be a lesson to him," Aronian said. AFP