GAIN Equine Nutrition, the Irish horse feed company, said on Friday https://www.gainequinenutrition.com/blog/news/statement-from-gain-equine-nutrition-2 evening that it was investigating "the potential presence of a contaminant in some batches of our equine feed range."
"Unfortunately the results of the urine samples taken from the horses have come back positive from the French laboratory," the Twitter site of O'Brien's Ballydoyle stable reported on Saturday.
"There is a possibility that the contaminant may have left their system by the time of racing tomorrow, 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 tomorrow."
O'Brien, arguably the world's most successful trainer who has won the Arc twice previously, has withdrawn his powerful quartet of entries Mogul, Japan, Serpentine and Sovereign.
His sons, trainers Joseph and Donnacha O'Brien, have also pulled out all the horses they were scheduled to run at the Longchamp meeting. In all, 11 horses trained by the family trio have been withdrawn. (Reporting by Hardik Vyas in Bengaluru; editing by Ian Chadband)