"On behalf of Frankie Dettori I can confirm that, as a consequence of a positive test at Longchamp on September 16 2012, he will be the subject of an inquiry by the Medical Committee of France Galop next week," said Christopher Stewart-Moore in a statement.
"In compliance with, and out of respect for, the regulations of France Galop, he will not be commenting further until the France Galop procedures have been completed."
The statement did not specify what the positive test was for.
France Galop was not immediately available for comment.
The 41-year-old Dettori, who enjoys celebrity status in the sport, announced in October that he was ending his 18-year stint as retained jockey for the Godolphin stable of Sheikh Mohammed, for whom he has ridden more than 100 Group One winners around the world since 1994.
Dettori memorably won all seven races in one afternoon at Ascot, England, in 1996, costing bookmakers millions of pounds in payouts to gamblers.