A second trainer from British racing headquarters Newmarket has been charged with giving anabolic steroids to horses, the British Horseracing Authority (BHA) announced on Friday.
Nine horses trained by Gerard Butler tested positive for steroids, but although he admits using the drug Sungate, which contains banned substances, he claims he did not realise he was in breach of the rules.
All nine horses have been suspended and Butler could face up to a 10-year ban if found guilty.
It is the second doping scandal to rock Newmarket this year, after Godolphin trainer Mahmood al-Zarooni was given an eight-year ban in April for using prohibited steroids on 11 horses.
A date and panel for Butler's hearing have yet to be announced.
In an interview with The Independent newspaper last month, Butler said more than 100 Newmarket-based horses may have been given Sungate after the product was recommended to him by vets.
"It did not cross my mind that there could be any problem with this medication," he said.
"And, judging from the fact that the BHA said nothing about it when they saw my medical book, it does not seem to have crossed their minds, either."
Anabolic steroids are banned in Britain, but they can be used out of competition in countries including Australia, Dubai and the United States.
The BHA announced an investigation earlier this week after obtaining positive samples during tests at Butler's yard in February.
The organisation said that a wider probe into the use of Sungate was also being carried out.
"The BHA has spoken to several trainers to establish the extent of the use of the product," the governing body said in a statement.
"Once all relevant information has been collated, a decision will be made as to how to proceed with this investigation. No definite timescale can be placed on this wider investigation at present."
Butler, 47, has trained a number of top horses including Elusive City and Compton Admiral, while he has also had success in the United States.
In 2011, he saddled Pachattack to finish third in the Ladies' Classic at the Breeders' Cup, the most prestigious fillies' race in America.
Al-Zarooni dropped an appeal against his ban from the BHA last week, following the most serious doping scandal in the recent history of British racing.