Graham Gibbons has voluntarily stood himself down with "immediate, indefinite effect" after the Kempton stewards reported the jockey to have swapped a urine sample with that of another rider 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.
They referred the matter to the British Horseracing Authority, who have launched an investigation.
Paul Struthers, chief executive of the Professional Jockeys Association, who was present at Kempton, issued the following statement:
"Callum made a serious error of judgement but to his credit very quickly took steps to correct that error, which ultimately led to the resulting stewards' inquiry into the matter, to which he gave candid evidence.
"The PJA can also confirm that Graham Gibbons has voluntarily stood himself down with immediate, indefinite effect.
"The PJA is in touch with both jockeys and will provide all necessary support. They have been assigned legal representation and will co-operate fully with the investigation."
Robin Mounsey, spokesman for the BHA, 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."