Aidan and Joseph O’Brien’s Australian runners have been tested by Racing Victoria after the duo were forced to pull out all their runners on Sunday’s Prix de l’Arc de Triomphe card due to an issue with contaminated feed.

A total of 10 horses across the two yards arrived in Melbourne on Friday night and are currently completing their two-week quarantine at the International Horse Centre in Werribee.

However, the runners, which include Melbourne Cup fancy Tiger Moth and Caulfield Cup hope Anthony Van Dyck, are all undergoing out-of-competition testing after it emerged their feed could have contained zilpaterol – potentially from batches of feed supplied by Gain Equine Nutrition.

The O’Briens pulled out their ParisLongchamp runners after they tested positive for the banned substance, and Racing Victoria has taken samples from the team’s horses to check whether they have also been affected.

A statement said: “Racing Victoria (RV) can advise that its Integrity Services team are liaising with Irish trainers Aidan and Joseph O’Brien regarding the circumstances that led to the stable scratching runners in France and Ireland across the weekend due to concerns regarding the alleged contamination of horse feed that it utilises.

“Reports from both trainers and international racing authorities are that feed used by both stables has been found in Europe to be contaminated with the prohibited substance zilpaterol.

“Zilpaterol is classified as a beta-agonist. Beta-agonists, such as the commonly used clenbuterol, are medications that primarily have an effect on an animal’s breathing. They are permitted for therapeutic use but are a prohibited substance in a horse’s system on raceday.

“For clarity, beta-agonists are not classified as anabolic steroids under the Australian Rules of Racing and thus the detection of a beta-agonist does not carry a mandatory stand down period. Beta-agonists are also not classified as a growth hormone under the Rules of Racing.

“Following discussions with the O’Brien stables, RV stewards have today conducted out of competition testing on their horses at Werribee to determine whether zilpaterol is currently in any of the horse’s systems. The results of these tests may take up to one week.

“Furthermore, both stables have confirmed to RV stewards today that they have ceased using feed from the provider in question.

“RV stewards will continue to work with the O’Brien stables regarding the situation and will make a further comment once the result of the out of competition tests are known and the stables have been notified.”