The BHA accused the employee of Dubai ruler and Godolphin owner Sheikh Mohammed bin Rashid al-Maktoum of a "deliberate flouting" of the rules and a "widespread systematic misuse of illegal substances".
It said he had also handed the drugs to unqualified staff at the stables in Newmarket, the headquarters of British flat racing, to administer them.
In a scandal that has caused serious embarrassment to Sheikh Mohammed, al Zarooni was banned last week after 11 horses in his care tested positive for anabolic steroids and he admitted administering prohibited substances to four others.
The BHA said that Al Zarooni had told its disciplinary panel that he had brought the steroids into the country when he flew back from Dubai.
Referring to the use of Stanasol, containing the banned steroid stanozolol, the trainer said that on March 14 he had "made up five unmarked syringes each containing 4ml of the drug from his bottle of the drug.
"He then drove to Moulton Paddocks Stables and passed the syringes out of his car window to an unqualified veterinary assistant, Sharif Mahboob, and asked him to give the drugs to five horses which he listed on a piece of paper.
"All five horses were under veterinary care at the time in respect of musculoskeletal problems and in one case, colic. Al Zarooni said he thought the drug would help improve the horses' condition."
The 15 horses al Zarooni admitted to doping, including leading 1,000 Guineas contender Certify, were banned from racing for six months.
Al Zarooni, who recognised he had made a catastrophic error, claimed that he had not realised what he was doing was not allowed in Britain, where anti-doping rules are strict.
In other countries, such as Australia and Dubai, the use of anabolic steroids is permitted out of competition.
The BHA noted, however, that he had not recorded the administration of any of the anabolic steroids in the stable's medication books and could offer no explanation for the omission.
It added that its own veterinary advisor was of the opinion that the drugs administered had "little or no therapeutic indication in horses" but were likely to be performance-enhancing in a similar way to human athletes.
Stanozolol is the steroid made notorious by disgraced Canadian Olympic sprinter Ben Johnson at the 1988 Seoul Games.
"Al Zarooni's assertion at the hearing that he did not know that such administration was not permitted in the UK was simply not truthful," the BHA report said.
"He asserted that he was only trying to do the best for his horses who were unwell. He did not have a credible explanation as to why he had not discussed the matter with the stable's veterinary surgeons or entered a record of the administration of the drugs in the stable's Medication Books.
"The Panel concluded that Al Zarooni sought to confer an unfair advantage on his horses by the underhand administration of illegal medication.
"His attempt at cheating was uncovered by the regulatory inspection and he had no justifiable excuse for his behaviour."
- Sports & Recreation
- anabolic steroids
- Sheikh Mohammed