A top US chess player accused of cheating after Magnus Carlsen quit a prestigious tournament has admitted to doing so as 12 and 16-year-old – but insisted that he was now “clean” and was even prepared to play naked to prove his innocence.

Hans Niemann, the 19-year-old who stunned the world champion Carlsen at the $500,000 Sinquefield Cup on Sunday evening, promised that he was a reformed character after revealing that he illegally benefited from computer assistance while playing chess on the internet as a child.

And in a passionate response, Niemann accused Carlsen and others – including the world’s best blitz player, Hikaru Nakamura – of trying to ruin his career.