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. He is scheduled to appear before the FFR anti-doping commission on August 21 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."
Toulon earlier issued a statement which read: "Toulon Rugby Club received a letter this Wednesday, July 25, 2012, from the French Rugby Federation reporting an 'abnormal' result from an anti-doping test taken by Steffon Armitage after the Top 14 final on June 9 at the Stade de France.
"Before the game, Steffon Armitage took two tablets of paracetamol codeine, a medicine strictly authorised by the French Agency for the Fight against Doping (AFLD).
"Having suffered with back pain in the week leading up to the game, Steffon Armitage was given anti-inflammatories as well as paracetamol codeine (two to four tablets per day) for these pains. All of the medicines administered are authorised by the AFLD.
"The presence of morphine in Steffon Armitage's urine sample is explained by the fact that 10% of this medicine (paracetamol codeine) is converted naturally into morphine.
"Steffon Armitage has absolutely not taken any illegal substance to improve his performances, but solely a medicine strictly authorised by the AFLD and well known to all."