Hans Niemann, 19, has previously admitted cheating twice in games when he was aged 12 and 16, but an investigation by chess.com has allegedly found more occasions, according to The Wall Street Journal.
Mr Carlsen, the number one player in the world, branded his rival a cheat last month after he withdrew from a tournament after losing to him, then resigned from an online match against him after one move.
The newspaper states that the report by chess.com, a platform where many of the world’s top chess players compete in online matches, alleges “that Niemann likely received illegal assistance in more than 100 online games, as recently as 2020. Those matches included contests in which prize money was on the line.”
The platform reportedly used a string of tools to detect the alleged cheating, including an analytics programme that compares human moves to those recommended by chess engines, “which are capable of beating even the greatest human players every time.”
The report states that some of the alleged cheating took place as recently as 2020, when Mr Niemann was 17 years old.
The Journal says that Mr Niemann “privately confessed to the allegations” and that he was “subsequently banned from the site for a period of time.”
The report noted that Mr Niemann’s improvement had been “statistically extraordinary” but did not make any conclusion as to any irregularities in his in-person games.
But it said that some of Mr Niemann’s strongest events “merit further investigation based on the data.” An investigation into Mr Carlsen’s claims is also being carried out by the sport’s governing body, FIDE.
“Outside his online play, Hans is the fastest rising top player in Classical (over-the-board) chess in modern history,” the report states.
“Looking purely at rating, Hans should be classified as a member of this group of top young players. While we don’t doubt that Hans is a talented player, we note that his results are statistically extraordinary.”
Mr Carlsen won the Julius Baer Generation Cup despite resigning against Mr Niemann, after which he said that he understood that his “actions have frustrated many in the chess community.”
“I’m frustrated. I want to play chess. I want to continue to play chess at the highest level in the best events,” he stated.
“I believe that cheating in chess is a big deal and an existential threat to the game. I also believe that chess organizers 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.”
Mr Carlsen stated that he had considered pulling out of the event when Niemann was invited to take part and was blunt in his allegations against his opponent.
“I believe that Niemann has cheated more — and more recently — than he has publicly admitted.”
Mr Niemann has insisted that he has never cheated in a live-streaming game. “I would never, could even fathom doing it, in a real game,” he said.
The chess.com report states that of the more than 100 suspect games, 25 were live-streamed and that there were several prize-money events.
The Independent has reached out to Mr Niemann for comment.