Five-time world chess champion Magnus Carlsen has directly accused American grandmaster Hans Niemann of cheating more than he has publicly admitted.
In an explosive statement posted to social media, the Norwegian said his unexpected loss to the 19-year-old earlier this month had strengthened his existing suspicions.
"When Niemann was invited last minute to the 2022 Sinquefield Cup, I strongly considered withdrawing prior to the event. I ultimately chose to play," Carlsen said.
"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.
"This game contributed to changing my perspective."
Carlsen sensationally withdrew from the $500,000 tournament the day after losing to Niemann.
Last week the pair met again in an online match, with Carlsen resigning after making just one move.
Niemann has vehemently denied cheating against Carlsen, offering to play naked to prove he receives no illegal assistance.
However, the teenage prodigy did admit to cheating in online games when he was 12 and 16.
"This is the single biggest mistake of my life and I am completely ashamed," Niemann said.
"I'm telling the world because I don't want misrepresentations and I don't want rumours."
Carlsen alleges Niemann has not told the full story.
"I believe that Niemann has cheated more — and more recently — than he has publicly admitted," he said.
"We must do something about cheating, and for my part going forward, I don't want to play against people that have cheated repeatedly in the past, because I don't know what they are capable of doing in the future."
Carlsen said he knows the way he has so far handled the situation has caused much angst within the chess community.
"I'm frustrated. I want to continue to play chess at the highest level in the best events," he said.
"I also believe that 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."
Last week Carlsen was rebuked by the International Chess Federation (FIDE) for his early resignation in his most-recent match against Niemenn.
"We strongly believe that there were better ways to handle this situation," it said.
"The world champion has a moral responsibility attached to his status, since he is viewed as a global ambassador of the game."
But FIDE said it was prepared to investigate the cheating allegations.
"FIDE is prepared to task its Fair Play commission with a thorough investigation of the incident, when the adequate initial proof is provided, and all parties involved disclose the information at their disposal," it said.
"It is our hope that this whole situation could have a long-term positive effect, if tackled properly."
