Last year's champion apprentice Marco Ghiani has been stood down from riding on an "interim basis" according to the BHA, with reports suggesting he failed a drug test and is under investigation.
British racing's governing body refused to comment when asked why the jockey was not available to ride but The Telegraph and The Sun report the 23-year-old was stood down at Brighton Sept. 5 after testing positive last week.
Ghiani was officially deemed 'unfit' to ride by the stewards at Brighton on Monday and subsequently missed two rides at Leicester on Tuesday.
Newmarket trainer Stuart Williams, for whom Ghiani rides regularly, said he had not been made aware of why the jockey was unavailable, while Ghiani's agent Sash Righton also said he did not know the facts regarding the investigation.
Ghiani refused to comment when contacted by the Racing Post.
A BHA spokesperson said on Wednesday: "The BHA does not comment on ongoing investigations or speculation around possible investigations."
Riders can face suspensions starting at two months or as long as half a year for a failed drugs test. If disciplinary action is deemed necessary, Ghiani's ban would start from Monday.
The jockey is considered one of the rising stars of the weighing room. He enjoyed Royal Ascot and group-race success with Real World in 2021 and has ridden 64 winners this year.