South Africa wing Aphiwe Dyantyi has tested positive for an unspecified banned substance but denies wrongdoing and has requested his B sample to analysed.
The 2018 World Rugby Breakthrough Player of the Year tested positive during a Springbok training camp last month and said he was determined to clear his name.
"I want to deny ever taking any prohibited substance, intentionally or negligently to enhance my performance on the field. I believe in hard work and fair play," Dyantyi said in a statement.
"I have never cheated and never will. The presence of this prohibited substance in my body has come as a massive shock to me and together with my management team and experts appointed by them, we are doing everything we can to get to the source of this and to prove my innocence.
"As a professional sportsman on national and international level, we get tested on a regular basis. I have been tested before and again, since this test.
"Taking any prohibited substance would not only be irresponsible and something that I would never intentionally do, it would also be senseless and stupid."
Dyantyi revealed he had tested positive on July 2 but had also undergone a test on June 15 which did not return a positive finding.
"I want to apologise in advance to my team mates and management at the Lions and Springboks, my friends and my family for the negative impact this news may have.
"Personally, this game has given me an opportunity to inspire not only the young rugby hopefuls but South Africans across all walks of life and that is something I do not take for granted and something I would definitely not risk by doing a stupid act like this."
Bok coach Rassie Erasmus will announce his Rugby World Cup squad on Monday but had already ruled Dyantyi out of contention for selection due to a hamstring injury.
The wing made his international debut against England in June last year and played 13 tests for the Boks, scoring six tries.