(CNN) -- The trainer at the center of a major horse doping scandal has been charged with multiple breaches of the rules by the British Horseracing Authority.
Employed by the ruler of Dubai's prestigious Godolphin operation-- Sheikh Mohammed bin Rashid Al Maktoum -- trainer Mahmood Al Zarooni has been summoned to attend a disciplinary hearing in central London on Thursday.
Testers from the BHA found anabolic steroids -- ethylestranol and stanozolol -- in 11 samples taken from 45 horses from Godolphin's Newmarket stable in eastern England.
Sheikh Mohammed said he was "locking down" the stables with immediate effect and ordering an internal review.
Anabolic steroids work on horses as they do on humans -- helping to build strength and muscle.
Al Zarooni, who has admitted using the banned substances in error, faces charges relating to the use of prohibited substances, keeping medication records and conduct "prejudicial to horse racing".
In addition to the 11 horses that tested positive on April 9, Al Zarooni in co-operating with the investigating officers also admitted that he had given steroids to four more horses. That quartet were not subjected to testing.
"As these additional horses were not tested and no positive samples exist, no charges are brought in relation to them (prohibited substances) and as such the trainer remains facing the original 11 counts of this charge at this stage," said the BHA.
"However, it is appropriate to charge the trainer with breaches of the rules related to medication records and conduct prejudicial to horse racing related to these horses, and therefore the trainer now faces 15 counts of these charges.
Earlier this week Al Zarooni said, in a statement by Godolphin, that he had a made a "catastrophic error."
He added: "I deeply regret what has happened because the horses involved were not racing at the time, I did not realize that what I was doing was in breach of the rules of racing.
"I can only apologize for the damage this will cause to Godolphin and to racing generally."
Godolphin is one of the world's most successful stables, with more than 2,000 race wins since it was formed in 1992.
It has been named Champion British Owner eight times, including last year, while Dubai ruler Sheikh Mohammed and his family have also taken the title on multiple occasions going back to the 1980s.
Meanwhile Sheikh Mohammed pledged to "lock down" Goldophin's Moulton Paddocks stables and test every horse at its Newmarket operation.
"I was appalled and angered to learn that one of our stables in Newmarket has violated Godolphin's ethical standards and the rules of British racing," he said.
"I have been involved in British horse racing for 30 years and have deep respect for its traditions and rules. There can be no excuse for any deliberate violation.
"Godolphin is fully cooperating with the British Horseracing Authority to get to the bottom of this matter and take any appropriate disciplinary action.
"I have ordered the Godolphin management to undertake an immediate review of our internal procedures and controls to ensure to prevent any reoccurrence of this type of activity in any stables of mine.
"We will be locking down the Moulton Paddocks stables with immediate effect, and I have instructed that I want a full round of blood samples, and dope testing done on every single horse on that premises.
"I can assure the racing public that no horse will run from that yard this season until I have been absolutely assured by my team that the entire yard is completely clean."
Zarooni joined Godolphin in March 2010 and won the St. Leger, one of Britain's five classics, in his first season.
But the sanctions now facing the 37-year-old at the disciplinary hearing include substantial fines, the withdrawal of his training licence or a suspension from the sport.
Stanozolol was still legal as recently as 2008, when Big Brown won the Kentucky Derby and the Preakness Stakes in the U.S. after having regular injections.
His trainer's admission, and the subsequent outcry, led to the drug being widely banned.
The BHA said there has only been one other case of anabolic steroids being found in a tested horse in recent years, which was when British trainer Howard Johnson allowed horses to run under the substances in 2008 and 2009.
The use of anabolic steroids is now prohibited at all times for any horse registered as "in training" under the care of a trainer licensed by the BHA.
However, it is still legal for out-of-competition use in Australia, such as helping to overcome injuries.
The BHA carries out testing during race meetings and while horses are in training.
In 2012, the organization tested 7,182 samples from a total of 90,174 runners -- 14 of which returned positive results. The BHA conducted between 600-700 tests in 2012 while horses were in training.