Graham Gibbons has been accused by the Kempton stewards of swapping a urine sample he was required to provide with that of another jockey at a meeting on Wednesday evening.
Gibbons had one ride on the card and was asked to provide the sample after finishing sixth on Button Up in the second division of a one-mile maiden event.
However, the stewards reported Gibbons had substituted his sample with that of fellow rider Callum Shepherd, who also had one ride at the meeting, with details of the incident having been passed on to the British Horseracing Authority.
A stewards' report on the BHA website said: "The stewards held an inquiry following a report from the clerk of the scales that Graham Gibbons, the rider of Button Up, had substituted the urine sample that he was required to produce with that of another rider, Callum Shepherd.
"Having heard evidence from the sampling officer, Graham Gibbons, Callum Shepherd, Paul Struthers, the chief executive of the PJA who accompanied Shepherd, and a valet, they forwarded a report to the head office of the British Horseracing Authority for further consideration."