South Africa wing Aphiwe Dyantyi faces a four-year ban after testing positive for performance-enhancing substances.
The 25-year-old - World Rugby Breakthrough Player of the Year in 2018 - failed a doping test on 2 July but requested a second sample be tested.
The South African Institute of Drug Free Sport (Saids) confirmed on Friday that the B sample also tested positive.
Dyantyi, not named in the Springboks squad for the World Cup, denies taking a prohibited substance.
The wing, who plays for the Golden Lions in Johannesburg, was provisionally suspended after the first sample was found to contain multiple anabolic steroids.
Dyantyi made his debut for South Africa against England in June 2018 and has 13 caps in total.
"SA Rugby, the Lions Rugby Company and Dyantyi are working with Saids, the World Anti-Doping Agency (Wada) and all other relevant authorities on the matter and no further comment can be made at this stage," a comment from South African rugby's governing body said.
Dyantyi can accept the charge and submit a plea, providing mitigating circumstances to reduce any sanction.
If he pleads not guilty, a hearing will take place before an independent panel in the next four weeks.