Last season’s champion apprentice jockey Marco Ghiani has been suspended from riding on an “interim basis” by the British Horseracing Authority.
The Italian was not permitted to take two rides at Brighton on Monday, with the stewards’ report stating the BHA considered him “unfit” to compete.
He missed further scheduled rides at Leicester on Tuesday and at Wednesday’s Kempton meeting.
Ghiani, 23, has not responded to reports he has failed a drugs test.
The Daily Telegraph and Sun newspapers reported he tested positive last week for a banned substance.
If a positive result is confirmed – it could be due to a medication – he would then face a disciplinary hearing.
A spokesman for the BHA said it “does not comment on ongoing investigations or speculation around possible investigations”.
The Professional Jockeys’ Association also declined to comment.