Frankie Dettori's failed drugs test in September was not for a performance-enhancing substance, the British-based Italian jockey's lawyer says.

Dettori, one of the outstanding riders of his generation, is now set to face a medical hearing before the medical committee of France Galop, French horse-racing's ruling body, after it was announced on Tuesday he'd failed a test taken during a meeting at Parisian course Longchamp on September 16.

Christopher Stewart-Moore, Dettori's lawyer, has told the BBC: "There are a large variety of substances which are banned by France Galop.

"We are not going to say which substance he has tested positive for as that would be in breach of their rules, but it's not a performance-enhancing drug."

The 41-year-old Dettori had four rides at the Prix de l'Arc de Triomphe trials meeting.

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, who came in third in the Prix Gladiateur.

It was after the ride on Farhh that he failed the doping control.

News of Dettori's failed test came three weeks after he split with Dubai-based Sheikh Mohammed's Godolphin set-up, which provided him with several major race winners, to end an 18-year association with the high-powered training and bloodstock operation.

This is not the first time flat jockey Dettori, who in 1996 made history when riding all seven winners during a day's racing 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.

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, receiving a six-month ban.