Twenty-three Chinese swimmers tested positive for the banned drug TMZ months before the Tokyo Olympics in 2021, it has emerged. However, they were cleared to compete at the Games after the World Anti-Doping Agency (Wada) accepted an explanation from the Chinese authorities that the kitchen at their hotel was contaminated.

The story, which has come to light after a joint investigation by the German TV channel ARD and the New York Times, has led to widespread criticism of Wada from senior figures in anti-doping, with one calling the case “shocking” and another a “devastating stab in the back to clean athletes”.

Several of the Chinese swimmers went on to win medals in Tokyo – including three golds. Many of them, including the two-time gold medallist Zhang Yufei, are expected to challenge again at the Paris Olympics.

ARD and the New York Times said they had seen a 61-page investigative report from the Chinese anti-doping agency, Chinada, to Wada in 2021, which said that it had found trace elements of TMZ in the extractor fan, on spice containers and in the drain of a hotel kitchen in Shijiazhuang, where the swimmers had been staying.

Chinada also pointed to low concentrations of TMZ, a heart drug that improves performance, in the urine samples of the swimmers as grounds to conclude intentional doping was “impossible”. According to the New York Times, it did not explain how a prescription drug available only in pill form had contaminated the kitchen.

Wada confirmed it had not appealed against the decision to clear the Chinese swimmers after reviewing the evidence. It said it had not been possible for its scientists or investigators to conduct inquiries on the ground in China “given the extreme restrictions in place” due to a Covid-related lockdown.

“Wada ultimately concluded that it was not in a position to disprove the possibility that contamination was the source of TMZ and it was compatible with the analytical data in the file,” it said.

“Wada also concluded that, given the specific circumstances of the asserted contamination, the athletes would be held to have no fault or negligence. As such, and based on the advice of external counsel, Wada considered that an appeal was not warranted.”

World Aquatics, which is in charge of global swimming, also said it was confident the positive tests were handled “diligently and professionally”.

However, it has emerged the International Testing Agency and the US Anti-Doping Agency contacted Wada to express concerns about the way the Chinese test results were reported.

ARD said it had spoken to a whistleblower in China who told them: “The whole story about contamination is a fairytale to me. None of the officials’ explanations are credible.” Meanwhile, the New York Times said it had spoken to five independent anti-doping experts who found the findings of the Chinese investigation “implausible”.

The head of US Anti-Doping, Travis Tygart, who said he had provided Wada with several allegations of doping in Chinese swimming since 2020, was also critical of how the case had been handled. “This appears to be a devastating stab in the back of clean athletes and a deep betrayal of all the athletes who compete fairly and follow the rules,” he said.

Meanwhile David Howman, a former director general of Wada who heads the Athletics Integrity Unit, said the case was “shocking”. He said: “What it would say to me immediately is that perhaps there was some form of programme in this sport to ‘prepare’ swimmers for the Tokyo Olympics.

“My concern is intense. You want the public to have confidence in your regulator. If you lose that confidence, then the reputation of the regulator starts going down the gurgler. And if that were to occur, that would be a tragedy for Wada.”

Olivier Rubin, Wada’s senior science and medicine director, insisted the case had been thoroughly reviewed in June and July of 2021. “Indeed, we even sought pharmacokinetic and metabolism information from the manufacturer of TMZ in assessing the plausibility of the contamination scenario that was presented to Wada,” he said. “Ultimately, we concluded that there was no concrete basis to challenge the asserted contamination.”

Chinada and the Chinese Swimming Association did not immediately respond to requests for comment.