Flat jockey Nathan Evans faces a likely ban after testing positive for a prohibited substance, understood to be cocaine.
Evans failed a drugs test after riding at York in August and has been stood down since the result became known, just before he was due to ride at Beverley a few days later.
Professional Jockeys Association chief executive Paul Struthers said: "Nathan has been informed by the BHA that he tested positive for a prohibited substance and was suspended with immediate effect as a result.
"Nathan is disappointed with himself for his mistake and is deeply sorry for his lapse of judgement. He has informed and apologised personally to those who have supported his career thus far.
"He has not asked for the B sample to be tested and is engaging with the PJA and BHA on the process that now follows."
Fellow jockeys William Carson, Kieran Shoemark, Callum Rodriguez and Kevin Lundie have all been banned for taking cocaine in the last two years.
Evans, 23, is originally from Northern Ireland and started out as a showjumper before turning to riding on the Flat at the age of 17.
He moved to Britain to join Mick Easterby's yard on a trial and has been there ever since, riding his first winner on Among Angels at Thirsk in April 2015.
He rode a personal-best 49 winners as an apprentice in 2016 and rode out his claim the following year.
Although he was sidelined for six months after a fall at Ripon in August 2017, he returned to action in February 2018 and rode the 100th winner of his career later that season.
He has ridden more than 150 winners in total, notably on Hoofalong in the Scottish Sprint Cup at Musselburgh and Hoof It in the Stewards' Sprint Stakes at Goodwood. However, he was also banned for ten days last year for striking a wayward horse across the muzzle at Doncaster.