The British Horseracing Authority has explained its decision to ban Godolphin trainer Mahmood Al Zarooni for eight years, saying he is guilty of a "widespread systematic misuse of illegal substances".

The 37-year-old was suspended last Thursday after he admitted administering banned anabolic steroids to 15 horses at the Gulf outfit's stables in Newmarket, eastern England.

Eleven of the horses tested positive and all were subsequently banned from racing for six months from April 9, the day they were tested.

Al Zarooni claimed he had made a "catastrophic error" and he did not realise he was in breach of racing rules by administering the drugs, as the horses were not racing at the time.

But the BHA said on its website: "This was a widespread systematic misuse of illegal substances which are absolutely prohibited under the rules.

"Nearly a quarter of the 45 horses tested at the stables had positive samples. These were horses in training, some of which were entered into races in April and May.

"The panel considered there was no excuse for Al Zarooni to be in any doubt as to the illegality of administering anabolic steroids...

"Al Zarooni's assertion that he did not know such administration was not permitted in the UK was simply not truthful."

The trainer's actions - described as the biggest doping scandal in racing in recent years - have prompted Godolphin to launch an internal investigation and shut its stables.

The BHA said Al Zarooni had brought the drugs himself into Britain on a flight from Dubai, where horses in training can be given anabolic steroids and race 28 days later.

"The panel concluded Al Zarooni sought to confer an unfair advantage by the underhand administration of illegal medication," the ruling said.

"He had access to expert veterinary advice from a number of sources but he deliberately ignored this resource and chose to covertly administer to horses in training anabolic steroids he had brought into the UK in his luggage from Dubai.

"The panel is firmly of the view that this was not an accidental or inadvertent misunderstanding of the rules - this was a deliberate flouting of the governance framework of British racing by one of the most high-profile Flat trainers working in the racing industry."