Jockey Callum Rodriguez is facing a lengthy ban from racing following a failed a drugs test.
The 21-year-old, based with County Durham trainer Michael Dods, rode 68 winners in 2018.
It is understood cocaine was found in Rodriguez's sample. He is suspended pending an investigation by the British Horseracing Authority.
"My one-off action was completely out of character and one that I deeply regret," said Rodriguez in a statement.
"I would like to sincerely apologise to all the owners and trainers I have ridden for, in particular Mr Dods and everyone at Denton Hall, as well as my fellow jockeys in the weighing room.
Rodriguez has partnered 12 winners from 64 runs so far this year but has not taken part in any races since 5 March at Wolverhampton.
He was the leading apprentice jockey on the northern circuit and third nationwide.
Dods said that he was "extremely disappointed" with Rodriguez's actions.
"As a yard, we operate a strict no-tolerance policy to drug taking, and each and every one of our staff are well aware of their responsibilities," he said in a statement.
"Through his actions, Callum has not only let himself down, he has also let down every member of our team, as well as owners and other trainers.
"We are fully supportive of the BHA's disciplinary process, and Callum will abide by whatever sanctions are eventually imposed.
"Callum is deeply remorseful for his actions and insists this was a one-off mistake. As his employer, we intend to work with him to help him atone for his error, along with the Professional Jockeys Association.
"Callum will now have some time in which to reflect on what he has done. He has to work hard and prove to everyone that he deserves another chance."