Bookmakers have promised to refund thousands of pounds in bets placed on horses that have tested positive for performance-enhancing drugs.
The news comes after one of the biggest doping scandals in British racing in which 11 horses from the Godolphin stable tested positive for anabolic steroids.
Godolphin trainer Mahmood Al Zarooni, who has admitted making a 'catastrophic mistake', will attend a British Horseracing Authority inquiry after irregularities were found in the 11 horses.
In a statement, Al Zarooni said: "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."
"This is a dark day for Godolphin," stable manager Simon Crisford told the BBC. "We are all shocked by what has happened."
Ladbrokes will return £200,000 worth of bets on horses that tested positive for steroids while William Hill and Coral will refund ante-post bets on four horses from the Godolphin stables - Certify, Desert Bloom, Artigiano and Restraint Of Trade.
A spokesperson for William Hill told Sky News: ''This is an unprecedented eventuality, and no-one betting could have predicted these events.
"We believe the fairest result for our customers is to refund their bets placed on the Godolphin runners.''
Al Zarooni faces a lengthy ban from the sport after admitting administering the drugs.