World chess champion Magnus Carlsen has for the first time openly accused fellow player Hans Niemann of cheating.
In a statement, he said he believed Mr Niemann had “cheated more — and more recently — than he has publicly admitted”, though offered no evidence.
Carlsen had previously made veiled accusations against Niemann, who beat him this month in a major upset.
Niemann, 19, has denied cheating in competitive chess and has accused Carlsen of trying to ruin his career.
The teenager has admitted cheating online twice, when he was 12 and 16, but has strenuously denied ever cheating over the board, and even said he was willing to play nude to prove his good faith.
The scandal began earlier this month after Mr Carlsen, considered by many to be the greatest player of all time, was defeated by Mr Niemann at the Sinquefield Cup, ending a 53-game unbeaten run in classical chess.
He subsequently quit the tournament, despite there being six rounds to go, and later tweeted a clip in which football manager José Mourinho is seen saying: “If I speak, I am in big trouble”.
Last week, the pair met again in an online tournament, but Mr Carlsen quit after making only one move – an apparent protest at Mr Niemann’s participation.
Mr Carlsen went on to win the tournament, and afterwards said he would be saying more about the scandal, adding that he wanted “cheating in chess to be dealt with seriously”.
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In a statement posted to Twitter on late on Monday, Mr Carlsen said: “I’m frustrated. I want to continue to play chess at the highest level in the best events.”
He went on: “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.
“I believe that Niemann has cheated more — and more recently — than he has publicly admitted.”
Mr Carlsen said he had become suspicious of Mr Niemann because he has made “unusual” progress in recent years.
He said that, during their game at the Sinquefield Cup, he felt Mr Niemann “wasn’t tense or even fully concentrating” while outplaying him using the black pieces “in a way I think only a handful of players can do”.
“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,” he said.
He added that there was more he wanted to say but couldn’t “without explicit permission from Niemann to speak openly”.
“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,” he said.
The BBC has contacted Mr Niemann’s representatives for further comment.
When the controversy erupted earlier this month, Niemann issued a strenuous denial, accusing Carlsen and others of trying to ruin his career.
“If they want me to strip fully naked, I will do it,” said Niemann.
“I don’t care, because I know I am clean. You want me to play in a closed box with zero electronic transmission, I don’t care. I’m here to win and that is my goal regardless.”
Grandmaster Nigel Short, the only British player to compete in the final of the world championships, told the BBC last week he was sceptical about the claims of foul play, saying there was no evidence Niemann cheated in his victory over Carlsen.
“I think in the absence of any evidence, statement or anything, then this is a very unfortunate way to go about things. It’s death by innuendo,” Short said.
