Toulon flanker Steffon Armitage has produced an "abnormal" doping test result - but his club insist he has not taken a banned substance.
The 26-year-old, who has played five times for England, was informed on Wednesday of the issue arising from a test taken after the Top 14 final against Toulouse at the Stade de France in June.
But a club statement released this morning insists the result was caused by approved painkillers and anti-inflammatories prescribed for back pain and that Armitage "has absolutely not taken any illegal substance to improve his performance".
Toulon revealed the prohibited substance present in Armitage's test was morphine, but said this is a natural side-effect of the painkillers taken by the former London Irish player.
The statement on the club's official website read: "Toulon Rugby Club received correspondence on Wednesday 25 July 2012 from the French Rugby Federation (FFR) reporting an "abnormal" doping test result that Steffon Armitage produced after the Top 14 final at the Stade de France on June 9.
"Prior to this match Steffon Armitage took two tablets of Dafalgan codeine, a medicine strictly authorised by the French Anti-Doping Agency (AFLD).
"Having suffered back pain in the week before the match, Steffon Armitage was prescribed anti-inflammatories such as Dafalgan Codeine (2 to 4 tablets a day) to ease the pain. All the medicines prescribed are authorised by the AFLD.
"The presence of morphine in the urine of Steffon Armitage is explained by the fact that 10 percent of this authorised medicine (Dafalgan codeine) is naturally processed into morphine.
"Steffon Armitage has therefore absolutely not taken any illegal substance to improve his performance, but simply taken a medicine that is strictly authorised by the AFLD and well known to all."
According to reports in France, Armitage will appear before the FFR anti-doping commission on August 21 and has been provisionally suspended pending the hearing.
Armitage won his first international cap against Italy in the 2009 RBS 6 Nations, following his brother Delon into the team - the first pair of brothers to represent England since wingers Rory and Tony Underwood.
But he has not featured since a game against the same opposition the following year and moved to France in 2011 after five seasons with Irish, who he joined after coming through Saracens' academy.
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."