World chess champion Magnus Carlsen said on Monday he believed Hans Niemann had "cheated more - and more recently - than he has publicly admitted", adding he no longer wanted to play against the American and any rivals who repeatedly cheated in the past.
The 31-year-old Norwegian resigned at the Julius Baer Generation Cup last week after one move against Niemann, who lags him by almost 200 Elo points - the rating system used to calculate the relative skill levels of players.
"So far I have only been able to speak with my actions, and those actions have stated clearly that I am not willing to play chess with Niemann," Carlsen said in a statement on Twitter.
Carlsen, world champion since 2013, also withdrew from the Sinquefield Cup in St Louis, Missouri earlier this month after unexpectedly losing to the 19-year-old, sparking a furore of comments and allegations that Niemann had cheated.
Niemann has denied any wrongdoing in over-the-board games.
"When Niemann was invited last minute to the 2022 Sinquefield Cup, I strongly considered withdrawing before the event. I ultimately chose to play," Carlsen said.
Niemann has previously been banned from chess.com for cheating online after admitting he had not played fairly in non-competitive games on the website in his youth.
"I believe that Niemann has cheated more - and more recently - than he has publicly admitted," Carlsen said on Monday.
"His over-the-board progress has been unusual, and throughout our game, in the Sinquefield Cup I had the impression that he wasn't tense or even fully concentrating on the game in critical positions while outplaying me as black in a way I think only a handful of players can do."
Niemann was not immediately available to comment.
"I have never cheated in an over-the-board game. If they want me to strip fully naked, I will do it. I don't care. Because I know I am clean," he said recently.
Carlsen also made a plea for stricter detection measures while repeating his concerns about cheating in the sport.
The International Chess Federation (FIDE) said last week it shared the Norwegian's concerns about cheating in the sport.
"Chess organisers and all those who care about the sanctity of the game we love should seriously consider increasing security measures and methods of cheat detection for over-the-board chess," Carlsen added.
"We must do something about cheating, and for my part going forward, I don't want to play against people that have repeatedly cheated in the past, because I don't know what they are capable of doing in the future."