The British Horseracing Authority has launched an investigation after the Kempton stewards reported Graham Gibbons to have swapped a urine sample with that of another jockey 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 Callum Shepherd, who also had one ride at the meeting.
Spokesman Robin Mounsey said: "The BHA cannot comment further regarding the incident at present as it is now an ongoing investigation.
"However, this is clearly an issue that we are taking very seriously and we will be taking the necessary steps to progress this matter."
The Kempton stewards inquired into the incident before referring the matter for further attention.
The official 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 BHA for further consideration."