The test on Sept. 16 during trials for the Prix de l'Arc de Triomphe at Longchamp found traces of cocaine, sources told the London tabloid The Sun.
"Frankie has failed a test for a banned substance which is thought to be cocaine," one source said. "This is obviously very serious for him."
Christopher Stewart-Moore, Dettori's lawyer, released a statement Tuesday saying his client is scheduled for a hearing next week before the medical committee of France Galop. Stewart-Moore said he and his client would not comment further until after the hearing.
Dettori, 41, is the son of Italian jockey Gianfranco Dettori. He has been Britain's Champion Jockey -- winner of the most races in a season -- on three occasions and has raced all over the world.
He lives near Newmarket, the center of British racing, and has been stable jockey for Godolphin Racing, the stable owned by the ruler of Dubai, for 18 years. He plans to become a freelance in 2013.