The British 100m sprinter CJ Ujah has been cleared of deliberately taking banned drugs by the Athletics Integrity Unit and the World Anti-Doping Agency, and will be free to return to competition next year.

Ujah led off Team GB men’s 4x100m relay team as they won silver at the Tokyo Olympics in 2021 only to test positive for two prohibited substances, ostarine and S-23. It led to the British team being stripped of their medals and Ujah facing a four-year ban.

However, on Monday, the AIU confirmed that Ujah would now be banned for 22 months after a thorough investigation found in his favour. He still has to serve a reduced ban as, under Wada’s strict liability rules, an athlete is responsible for everything they put in their body.

Speaking exclusively to the Guardian after this newspaper broke the news, Ujah revealed that it was a contaminated beta alanine supplement – bought off Amazon for £10 during lockdown – that caused his positive test in Tokyo. “Obviously I made a mistake,” he said. “But people make mistakes. I am not a cheat.”

Ujah also acknowledged that he did not check the brand he chose was approved by Informed Sport, which guarantees a product is batch-tested for prohibited drugs before going on sale, and therefore he had no defence when he tested positive. “I think complacency set in,” he said. “During the pandemic I relied a lot on Amazon, rather than using the people and resources around me. It was just convenient, with next-day delivery. And I didn’t think anything was wrong with it.”

“Many athletes see supplement education as a box‑ticking exercise,” he added. “And while I’m not going to call anyone out, a lot of athletes use supplements that are not Informed Sport because they don’t think anything bad could happen to them – until it does.”

In a statement the AIU confirmed that it accepted Ujah’s positive test was not caused by a deliberate attempt to cheat: “The AIU and Wada were satisfied that the sprinter’s anti-doping rule violation was not intentional as a result of his ingestion of a contaminated supplement and the applicable two-year period of ineligibility was reduced by two months on account of how promptly he admitted the violation,” it added.

The decision means Ujah will be free to compete again on 5 June 2023, giving him two months to prove his fitness before potentially returning to the British team in time for the world championships in Hungary.

However, the head of the AIU, Brett Clothier, warned athletes to understand the consequences that could arise from taking supplements and to pay better attention to the relevant rules and athlete education programmes.

Clothier said: “In this case, after a thorough examination of the facts, we were satisfied that Mr Ujah did indeed ingest a contaminated supplement, but he was unable to demonstrate that he was entitled to any reduction in the applicable period of ineligibility based on his level of fault.

“Taking supplements is risky for athletes as they can be contaminated or even adulterated with prohibited substances. Athletes owe it to their fellow competitors to be 100% certain before putting anything into their body. If there’s the slightest doubt, leave it out.”

The British relay team automatically forfeited their Olympic silver medals in February when Ujah did not challenge his adverse analytical finding at a court of arbitration for sport hearing