The details first emerged in the New York Times,, external which shared reporting with German broadcaster ARD.
The World Anti-Doping Agency (Wada) confirmed to the BBC it was informed of China Anti-Doping Agency’s (Chinada) decision not to punish the swimmers in June 2021.
It said it was “not in a position to disprove the possibility that contamination was the source of TMZ”.
China’s 30-member swimming team won six medals at the Tokyo Olympics, including three golds.
United States Anti-Doping Agency (Usada) chief executive Travis Tygart said Wada and Chinada had “swept these positives under the carpet” – claims Wada described as “completely false and defamatory” while adding that it had referred the comments to its lawyers.
“Aquatics GB believes that every athlete is entitled to compete on a level playing field – and that means a commitment to clean sport,” Aquatics GB said.
“Delivery against this commitment requires a testing process that is robust, transparent and consistently applied.
“As we build towards Paris 2024 we are fully supportive of the consistent and comprehensive testing process that our athletes have to follow as a means of keeping sport clean.”