Frankie Dettori faces a French horse racing inquiry after a "positive test" at French track Longchamp in September.
The jockey's solicitor, Christopher Stewart-Moore, said the inquiry, by France Galop, would begin next week.
In a statement, Stewart-Moore said: "As a consequence of a positive test at Longchamp on 16 September 2012, he will be the subject of an inquiry by the medical committee of France Galop."
The British-based Italian, 41, had four rides at Longchamp on that day.
The Sunday meet was a day of major trials for October's Prix de l'Arc de Triomphe meeting.
Dettori rode Marco Botti's Joshua Tree to third in the Qatar Prix Foy, while the John Hammond-trained Sarah Lynx trailed home last in the Prix Vermeille.
He was then beaten by a head into second on Godolphin's Farhh in the Prix du Moulin and was third for his former employers on Willing Foe in the Prix Gladiateur.
France Galop have not commented on proceedings and in keeping with similar previous cases, they have not issued a statement before the hearing.
"Whatever the outcome, Dettori is such a huge name so there'll be much interest in this hearing next week," said BBC Radio 5 live racing correspondent Cornelius Lysaght.
"If found in breach of any rules in this area jockeys can normally expect automatic suspensions."
Last month, three-time champion jockey Dettori announced he is to split with Sheikh Mohammed's Godolphin operation after an 18-year association.
Dettori, who rode all seven winners on the card at Ascot in September 1996, has won 14 British Classics, including the Epsom Derby on Authorized in 2007.
He is also a three-time winner of France's prestigious Prix de l'Arc de Triomphe, on Lammtarra in 1995, Sakhee in 2001 and Marienbard the following year.