Chess superstar Magnus Carlsen openly accused American Hans Niemann for the first time of cheating on Monday, saying the rising star had done so more recently and more often than he had previously admitted.

The chess world has been rocked by the Carlsen-Niemann controversy for three weeks since the 31-year-old five-time world champion withdrew from the Sinquefield Cup in the US after losing to Niemann, 19.

Last week, the Norwegian’s abrupt withdrawal from a match against Niemann in the Julius Baer Generation Cup online tournament caused a new sensation.

In a letter to Dear Chess World published on Twitter, Carlsen said: “I believe that Niemann has – and has lately – cheated more than he has publicly admitted.

“His progress on the board has been unusual.”

He didn’t specify the exact form of the alleged fraud, but added that there was “more I want to say”.

Niemann has cheated online twice, aged 12 and 16, but says he has never cheated in a face-to-face match and is even willing to play naked to prove his goodwill.

In early September, the giant chess platform chess.com blocked the American for cheating on the site.

AFP contacted Niemann for comment, but he did not immediately reply.

During their Sinquefield Cup match, Niemann didn’t appear tense or even “fully focused on the game in critical positions while outplaying me as a black guy in a way I think few players can,” Carlsen continued in his letter away.

He called cheating in chess “an existential threat to the game” and said “I don’t want to play against people who have repeatedly cheated in the past.”

He concluded by saying he hoped the truth would come out, “whatever it may be.”