Aidan O'Brien has withdrawn all four of his horses from Sunday's Prix de l'Arc de Triomphe in Paris after they tested positive for a banned substance.
It is suspected that the prohibited substance was in contaminated feed.
The Irish trainer was due to run Mogul, Japan, Serpentine and Sovereign in Europe's richest race, which Enable is bidding to win for a record third time.
His sons Joseph and Donnacha have also withdrawn horses they were due to run in other races at the meeting.
A total of 11 horses trained by the trio will be absent.
"Unfortunately the results of the urine samples taken from the horses have come back positive from the French laboratory," said a statement from O'Brien's Ballydoyle Racing base.
"There is a possibility that the contaminant may have left their system by the time of racing on Sunday, however we have no guarantee of this.
"In order to protect the integrity of racing we have decided to withdraw all our horses from racing (on Sunday).
"Joseph and Donnacha O'Brien will do likewise."
British trainer Roger Varian, who uses the same feed supplier, withdrew his seven intended runners from meetings on Saturday.
