Former Bristol Bears lock Chris Vui says he is “devastated” that the four-year ban he was issued with for a contested doping violation has been upheld by the Court of Arbitration for Sport [CAS].

Vui, 33, tested positive in August 2023 for 19-norandrosterone, an indicator for the steroid nandrolone, before that year’s Rugby World Cup, in which he represented Samoa.

He was immediately suspended by World Rugby but has always denied knowingly using the banned substance.

In a statement sent to BBC Sport, Vui said he hoped his “experience contributes to a broader discussion within sport about the operation of anti-doping processes”.

“Over the past three years, since being provisionally suspended during the World Cup, I have been fighting to prove my innocence, so I am devastated by this outcome,” Vui’s statement said.

“I want to make it clear I have never knowingly used, and would never knowingly use, a prohibited substance. It goes against everything I stand for as a person and as an athlete.

“The low level of the substance found in my test is consistent with contamination, and I therefore believe the most likely explanation is the positive test was caused by a contaminated supplement. Sadly, I could not prove this to a sufficient degree of certainty.

“This has been a deeply challenging and unexpected experience that has effectively put my life on hold. One of the hardest realities is, despite years of investigation and proceedings, I may never know for certain how this situation arose.

“For now, my focus is on returning to rugby next year and on what comes next as I continue to reflect on this chapter.”

Vui and his lawyers are unhappy about the way the process of testing his B sample [second sample] was carried out in a laboratory in Utah – namely that the doctor attending on his behalf was not present for certain stages of the process.

“During the process, my team and I also uncovered various issues with the laboratory procedure, which even the panel who heard my appeal described as ‘troubling’,” his statement added.

However, the CAS verdict ruled that the doctor’s absence from parts of the testing process “does not… render the results of the B sample invalid”.

Vui, who joined the Bears in 2017 and made more than 100 appearances for the club, had signed a contract with Bristol in February 2022 to run until 2026.

But it was annulled in September 2023 following the positive test and he has been unable to play professionally since then despite a series of appeals against the findings.

He also claimed his defence had been prejudiced by delays as he was not notified of a positive test until 57 days after he had provided the urine sample, thus he was unable to test the supplements he had been using as they were no longer available.

A full investigation was carried out by CAS and a 65-page statement, external was released on their website on Thursday.

They backdated the ban by several weeks to take into account the aforementioned delays but ultimately agreed with the decision made by World Rugby back in 2023.