Steffon Armitage and Toulon are confident the flanker will clear his name after a doping test returned an "abnormal" result.
The 26-year-old tested positive for the prohibited substance morphine after the Top 14 final defeat by Toulouse on June 9, the French Rugby Federation (FFR) confirmed in a letter to the club.
Armitage is scheduled to appear before the FFR anti-doping commission on 21 August and has been provisionally suspended pending the hearing.
Toulon issued a statement insisting the result was caused by a side-effect of painkillers and anti-inflammatories prescribed for back pain, which they say are approved by the French Anti-Doping Agency (AFLD).
Armitage, who has played five times for England, said in a press conference broadcast on the club's website: "It's a bit stressful, especially for my family in England.
"But I will keep my head held high because I have done nothing wrong. I know that everything will soon be sorted, that I will be able to forget about this and come back stronger."
