Chinese Olympic swimmer Sun Yang has been banned from the sport for eight years after destroying a sample of his blood intended for drug testing. The ban makes Sun Yang ineligible for the 2020 Tokyo Summer Olympics, and could end his career.

The ban centers on a September 2018 incident in which Sun's mother instructed his security guard to smash a vial of Sun's blood with a hammer before inspectors could take it for testing, The Associated Press reports.

Sun testified at a public hearing in November that the officials sent to retrieve the samples in 2018 did not produce proper identification, The Associated Press reports. An expert from the World Anti-Doping Agency disputed Sun's claim at the hearing.

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Sun, 28, became the first Chinese swimmer to win an Olympic Gold medal at the 2012 London Olympics, CBS Sports reports. He won gold again at the Olympic games in Rio de Janeiro in 2016, and has a total of 11 world championships to his name.

In an unanimous decision on Friday, the Court of Arbitration for Sport ruled that Sun tampered with his samples, preventing them from being tested. The three-person panel handed down an 8-year suspension.

"The athlete failed to establish that he had a compelling justification to destroy his sample collection containers and forego the doping control when, in his opinion, the collection protocol was not in compliance with the ISTI," the court wrote in its decision.

"As the Panel noted, it is one thing, having provided a blood sample, to question the accreditation of the testing personnel while keeping the intact samples in the possession of the testing authorities; it is quite another thing, after lengthy exchanges and warnings as to the consequences, to act in such a way that results in destroying the sample containers, thereby eliminating any chance of testing the sample at a later stage."

Sun told the Xinhua News Agency that he plans to appeal the ruling. "This is unfair. I firmly believe in my innocence," he told Xinhua. "I will definitely appeal to let more people know the truth."

The World Anti-Doping Agency welcomed the court's ruling. It said in a statement Friday it "is satisfied that justice in this case has been rendered."