Frankie Dettori, one of the outstanding jockeys of his generation, failed a drugs test when riding at Longchamp in France in September, his lawyer announced on Tuesday.
The legendary Italian rider's positive result will be the subject of a hearing conducted by France Galop, French racing's governing body, in Paris next week.
Dettori's solicitor, Christopher Stewart-Moore, in a statement to the British Press Association, reported: "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.
"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."
No details of the substance in question were forthcoming with France Galop telling AFP they would be making no comment "for the time being".
The 41-year-old Dettori had four rides at the Prix de l'Arc de Triomphe trials meeting on September 16.
He rode Joshua Tree to finish third in the Prix Foy, Sarah Lynx who trailed home last in the Prix Vermeille, then Farhh who was beaten a head in the Prix du Moulin and Willing Foe, another Godolphin charge, who came in third in the Prix Gladiateur.
It was after the ride on Farhh that he failed the doping control.
Tuesday's news comes three weeks after Dettori's split with Sheikh Mohammed's Godolphin operation after an 18-year association.
This is not the first time the celebrated horseman, who in 1996 made racing history when riding all seven winners at Ascot, has hit the headlines for the wrong reasons.
In 1993 he was stopped by police on London's Oxford Street with a small quantity of cocaine in his car.
He escaped with a caution but the incident cost him a lucrative two-year contract to ride in Hong Kong.
"It was the kick up the backside I needed," he admitted afterwards.
"I was going off the rails. It was too much too young - too much success, too much money, too much partying. I'd be late in the mornings. I didn't do what I was told. I was naughty.
"But when you lose everything, your survival instincts take over. I stopped being lazy. I made an effort and started to work with my talent."
Dettori, married with five children, went on to scale the heights of his profession.
He has won all five English classics, including the Epsom Derby on Authorized in 2007, and the Prix de l'Arc de Triomphe, Europe's most prestigious race, three times on Lammtarra in 1995, Sakhee in 2001 and Marienbard the following year.
Dettori is not the first British-based jockey to fall foul of French racing's doping rules.
In 2006 six-time British champion Kieren Fallon was banned for six months after testing positive for a metabolite of a prohibited substance.
The following season he failed a control for cocaine at Deauville racecourse and was suspended for 18 months.
Former top British jump jockey Dean Gallagher was found with the recreational drug in his system on three separate occasions in France in 2000, picking up a six month ban.