Doubt has been cast on the participation of Aidan O'Brien's runners in Sunday's Arc at Longchamp due to batches of Gain Feeds' products being contaminated with a prohibited substance. 

Gain released a statement this evening confirming that some of their feed has been contaminated with the growth agent Zilpaterol, which is not permitted in Europe.

"Until further investigations are completed, Gain customers are advised to refrain from feeding our equine products to their animals," the company advised.

"The contaminant, Zilpaterol, is a synthetic beta-agonist approved for use as a performance-enhancer in some beef production systems outside of the EU.

"It is important to stress that this synthetic substance has never formed part of any formulation in any of our animal nutrition ranges."

Irish champion trainer O'Brien has sent blood samples from his planned Arc runners, Mogul, Japan, Serpentine and Sovereign, for testing but the results are not expected until the eve of the race.

"They say after a day and a half or two days, the substance should be gone out of the horse's system," the Ballydoyle supremo told The Racing Post.

"So, if, when we get the results, there is a trace of the substance in it, we'll have to see how much is there and then make a decision, after changing the feed for two days, whether we should run on Sunday or not.

"The experts are telling us that it should be clear by then, so we'll just have to see."