Gerard Butler will be disqualified from the sport for five years after the British Horseracing Authority accused the trainer of an "appalling" dereliction of his duties.
The results of a BHA disciplinary panel held last month were released on Wednesday, and the news was revealed he had admitted to all seven charges against him relating to samples of an anabolic steroid found in horses in his care.
Nine horses produced positive samples, five cases of which were identified as the joint treatment Sungate, which contains the banned anabolic steroid stanozolol.
However, considered even more serious was Butler's admission of administering another substance, Rexogin, to four horses himself.
Rexogin is designed for use in humans, often for bodybuilding, and contains 10 times as much stanozolol as Sungate.
Adam Brickell, director of integrity, legal and risk for the BHA, said: "BHA's position, which was upheld by the disciplinary panel, was that the most serious charges related to Gerard Butler's gross failure to look after the best interests of four horses in his care, which amounted to conduct that was seriously prejudicial to the integrity, proper conduct and good reputation of the sport.
"The gravity of the breaches of the rules of racing escalated when, in the course of cross-examination during the disciplinary panel hearing, Gerard Butler finally provided evidence as to where he had purchased the drug in question from, and admitted that the product he had administered himself to four horses was not the equine veterinary product Sungate, but instead an unlicensed stanozolol-based product called Rexogin, manufactured for use in humans.
"Furthermore the panel accepted that Butler had administered this product by intra-articular injection using a method restricted by law to qualified veterinary surgeons.
"The panel also pointed to the fact that Butler took no veterinary advice before carrying out these procedures, did not have the horses properly assessed prior to their treatment, made no recording in his medication records of having injected the horses and that he subsequently allowed the horses to be treated by veterinary surgeons without informing them of the prior administrations.
"Furthermore they noted that it is not appropriate for a trainer to say he is able to undertake an invasive veterinary procedure on the basis that he had seen veterinary surgeons performing the procedure, that he created unnecessary risks for the horses in obtaining the drug from an unlicensed source and that his behaviour in administering the injections was consistent with the underhand and covert manner in which he purchased the drug.
"Taking this all into account the panel summarised that the actions of Butler represented 'an appalling breach of his duty to look after the interests of the horses in his care and amounted to conduct that was seriously prejudicial to the integrity, proper conduct and good reputation of horseracing in Great Britain'."
Brickell went on: "The charges also related to five other positive samples taken from horses which were administered Sungate on the advice of and by a veterinary surgeon.
"The rules are clear that the licence holder, in this case Gerard Butler, is wholly responsible for the presence of prohibited substances in horses in his care and control.
"However, it remains of concern to BHA that a practising vet regularly treating racehorses, and therefore presumably familiar with the rules, should have recommended and administered such a product to a horse in a trainer's care or control.
"This is an area we are continuing to explore and consult on with a view to strengthening our position. If the Royal College of Veterinary Surgeons chooses to follow up on this specific matter we would assist as required, as we have done on previous occasions."
Butler also admitted to failing to keep a record of treatments. His ban begins tomorrow and he will be suspended until December 4, 2018 inclusive. He has a period of 48 hours to arrange the relocation of his horses.