Gerard Butler's Newmarket yard is currently at the centre of a British Horseracing Authority investigation, it has been confirmed.

Butler told the Independent that, in what he described as an "unpardonable misjudgment", four of his horses had been treated with Sungate, a joint treatment which contains a banned substance, on the advice of his vet.

His admission comes hot on the heels of the BHA's decision to ban trainer Mahmood Al Zarooni for eight years after he admitted to using anabolic steroids on some of his horses.

The BHA confirmed some of Butler's horses returned positive results after random tests in February and it is trying to establish how many horses have been affected, with Butler suggesting other yards in Newmarket may have used the product.

A BHA statement read: "It is the general policy of the BHA not to comment publicly regarding ongoing investigations or speculation surrounding potential investigations.

"However, in light of reports and speculation today, and because of recent events regarding horses formerly trained by Mahmood Al Zarooni, it is felt necessary to confirm that a separate investigation is being held into a number of positive samples obtained from horses at Gerard Butler's yard, following a testing in training visit on February 20.

"While conscious of the need not to prejudice the outcome of the current inquiry, the investigation has established that the source of the positive samples was a veterinary product, licensed in the EU and legally imported for use by a veterinary practice, the initial administration of which was recommended by a vet.

"This investigation remains ongoing and a number other parties have been and will be interviewed, including representatives of the veterinary practice in question. One of the objectives of this investigation is to clarify the extent to which this product has been distributed and administered to horses in training.

"Immediately following the results of the testing in training, the BHA, in conjunction with the National Trainers Federation, notified trainers that the product in question contains an anabolic steroid and should not be used on any horse in training."

Fellow Newmarket trainer John Berry believes it would be "very, very sad" if Butler were to lose his licence over the incident.

"I think if what Gerard has said in the papers is correct, all he is guilty of is taking bad advice," he said.