The powerful racing outfit Godolphin Operation is under a cloud after it was revealed 11 of their horses had tested positive for anabolic steroids.
Godolphin, the brainchild of Dubai ruler Sheikh Mohammed al-Maktoum, said that it was a dark day for the team and an urgent review would be carried out.
All 11 horses who tested positive came from Mahmood Al Zarooni's yard in Newmarket, England.
The trainer - who has made a big impact since becoming one of Godolphin's trainers winning last year's Dubai World Cup and the oldest classic the English St Leger - said he had made a catastrophic mistake.
A costly one as well as all the horses who tested positive are barred from racing until further notice by the British Horseracing Authority (BHA), British racing's governing body, who will also hold a disciplinary hearing into the case.
Among those who tested positive was unbeaten filly Certify, who was vying for favouritism for next week's English 1000 Guineas, the first fillies classic of the season and run over a mile.
"I deeply regret what has happened. I have made a catastrophic error," Zarooni told the Operation's website.
"Because the horses involved were not racing at the time, I did not realise that what I was doing was in breach of the rules of racing.
"I can only apologise for the damage this will cause to Godolphin and to racing generally," added the 37-year-old, who has largely superseded Godolphin's first trainer Saeed bin Suroor as their main handler.
A statement on the Godolphin website read: "Following an inspection of Zarooni's stable by officials from the BHA, traces of prohibited substances were discovered in a number of the horses tested, including Certify. The BHA has advised that, as a result, the filly will not be allowed to take part in the Qipco 1000 Guineas at Newmarket next month.
"Al Zarooni has admitted that he was responsible for the administration of the prohibited substances."
Godolphin racing manager Simon Crisford said that Sheikh Mohammed had ordered a full internal review be carried out.
"This is a dark day for Godolphin. We are all shocked by what has happened.
"His Highness Sheikh Mohammed was absolutely appalled when he was told and this is completely unacceptable to him. We will await the outcome of the BHA inquiry before taking any further internal action.
"Sheikh Mohammed has instructed me to begin an urgent review of all of our procedures and controls. That is already under way and we will take advice from the BHA in completing it."
The BHA said that on April 9 this year samples were obtained from 45 horses trained by Al Zarooni at Moulton Paddocks Stables and that on Monday afternoon it had received written advice from the Horseracing Forensic Laboratory that upon analysis 11 of the samples had present in them prohibited substances, namely ethylestranol and stanozolol.
Adam Brickell, Director of Integrity, Legal and Risk for the BHA, said: "Ethylestranol and stanozolol are anabolic steroids and therefore Prohibited Substances under British Rules of Racing, at any time - either in training or racing. Mahmood Al Zarooni has been advised of the analysts' findings and has been visited by an Investigating Officer.
"A Disciplinary Panel inquiry into the analysts' findings will take place at the first available opportunity, confirmed details of which will follow when available.
"The horses which have produced positive tests will also not be permitted to race with immediate effect and for an extended period of time. As part of the ongoing process a decision will be made as to what period this suspension will be imposed for.
"The BHA understand the importance of this process being carried out as quickly as possible because of implications for betting markets."
A date for the hearing has yet to be announced.