Gerard Butler’s Newmarket yard is at the centre of a British Horseracing Authority investigation into banned steroids.

Butler, who trained at Churn Stables at Blewbury, near Didcot, for nine years up to May 2008, told the Independent that 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.

It is trying to establish how many horses have been affected, with Butler suggesting other yards in Newmarket may have used the product.

Butler described Sungate as “misunderstood by many others” and underlined the fact the BHA had not commented on its inclusion in the yard’s medical book.

He told the Independent: “I have been totally candid throughout, and it was I who told the BHA that I had treated four colts in December and January.

“I’m not trying to defend myself, just to explain what happened. And I must emphasise I was advised in good faith by my vets.

“It was an unpardonable misjudgement, purely to cut corners in what is a very expensive treatment.”