Jockey Kieren Fallon has failed a drugs test after a B sample tested positive, says his solicitor.
Fallon walked free from the Old Bailey in December 2007
The Irishman, 42, will have a hearing before French racing authorities next month and could face a lengthy ban.
Fallon tested positive in August for a banned substance after Myboycharlie's win at the Prix Morny in France.
"I had a letter at the weekend that confirmed the positive findings," his lawyer Christopher Stewart-Moore told the Times.
"The letter tells me that the Medical Commission wish to meet in January to consider the case.
"I have instructed our French legal counterparts to request all the relevant data before we decide on our next step."
Stewart-Moore has previously said some aspects of the test result were "curious".
The news that Fallon had tested positive for a banned substance, believed to be cocaine, emerged the day after his race-fixing trial collapsed.
The six-time champion jockey was acquitted of conspiracy to defraud at the Old Bailey on 7 December.
If convicted of a second offence in such a short space of time the authorities will take a very dim view of it
5 Live racing correspondent Cornelius Lysaght
Fallon was also given a six-month ban by France Galop, the governors of racing in France, in December 2006 after testing positive for a metabolite [produced by the body after it has processed a chemical] of a prohibited substance five months earlier at Chantilly.
The suspension effectively meant he could not ride on any racecourse in the world, as other governing bodies have a reciprocal suspension policy in such cases.
He could face another lengthy spell on the sidelines, with previous similar cases earning suspensions of a year to 18 months.
"If convicted of a second offence in such a short space of time the authorities will take a very dim view of it," said BBC Radio 5 Live racing correspondent Cornelius Lysaght.
"He could be out for the whole of 2008 and the most part of 2009 as well."
Fallon is 43 in February and a lengthy ban could put his position as number one jockey to the powerful Coolmore operation, and trainer Aidan O'Brien, in jeopardy.
Coolmore trio John Magnier, Michael Tabor and Derrick Smith stood by the rider throughout the race-fixing case, but there have been suggestions Johnny Murtagh might be a possible replacement.
Fallon had been banned from riding in the UK by the British Horseracing Authority from July 2006 pending the outcome of his race-fixing trial, but was free to compete in his homeland and France.
In October 2007, on the eve of his trial, he partnered Dylan Thomas to victory in the Prix de l'Arc de Triomphe at Longchamp in France.