South Africa wing Aphiwe Dyantyi is facing a four-year ban for doping after testing positive for multiple anabolic steroids.
The South African anti-doping agency says Dyantyi's "B'' sample confirmed the results of a test he failed at a Springboks training camp on July 2.
Dyantyi won World Rugby's breakthrough player of the year award in 2018.
The South African Institute for Drug-Free Sport said Dyantyi tested positive for three banned substances - metandienone, methyltestosterone and LGD-4033.
Dyantyi, 25, had maintained his innocence and requested that his B-sample be tested.
He said: "I want to deny ever taking any prohibited substance, intentionally or negligently.
"I have never cheated and never will. The presence of this substance in my body has come as a massive shock to me and my team are doing everything we can to get to the source of this and prove my innocence.
"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 missed out on South Africa's World Cup squad, having not played for his country since November last year.
He has scored six tries in just 13 appearances for South Africa, including two in the victory over New Zealand in Wellington.