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 a match against Niemann on Monday night in the “Julius Baer Generation Cup” online tournament, Carlsen resigned the game 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 US after losing to Niemann, a young rising star in the chess world but who has admitted to cheating in online games in the past.

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

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

The Norwegian’s latest withdrawal has reignited the controversy.

“It’s an unprecedented case,” Dutch chess master 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,” he added.

“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.