Snow Fairy has had her victory in the Prix Jean Romanet at Deauville in August taken away after failing a test for a banned substance, trainer Ed Dunlop said in a statement.
Connections have explained it was due to anti-inflammatory treatment the five-year-old has been receiving.
Snow Fairy has struggled with injury through her career, winning again in the Irish Champion Stakes after her comeback in Normandy, but she is currently on the sidelines again.
"We have sadly learnt that Snow Fairy has been disqualified from her victory in the Prix Romanet in Deauville as a result of failing a routine dope test taken after the race," said Dunlop.
"Due to the significant tendon injury that she suffered at the end of last year, Snow Fairy has required anti-inflammatory medication to help the injury.
"Unfortunately the withdrawal period was longer than we had been advised. The trainer is ultimately responsible and for that we would like to offer our sincerest apologies to all concerned parties."