Oisin Murphy's comeback from a three-month ban has been delayed due to what he described as an “administrative error.”
However, the British Horseracing Authority said the two-time champion rider had not completed a course of testing required beforehand.
Murphy planned to race-ride for the first time in 128 days at Lingfield on Friday.
The 25-year-old was banned after a drugs test at Chantilly in July showed metabolites of cocaine.
He always strenuously denied taking the drug and requested a B sample.
France Galop held a hearing where the rider’s defence of environmental contamination from a sexual encounter and scientific hair test evidence was accepted.
Murphy, also champion apprentice in 2014, was set to have three mounts at Lingfield this afternoon, beginning with Rain Gauge in the mile novice event (1.30).
He was also booked by five sets of connections at Chelmsford on Saturday.
Murphy tweeted: "At 18.50 yesterday I was personally informed by the BHA due to an administrative error I will not be able to partner my rides at Lingfield today and Chelmsford on Saturday.
“I’m currently looking into this situation with the BHA and I plan to be back on the racetrack as soon as I possibly can.
"I apologise profusely to all the owners, trainers and supporters who I’m having to let down at such late notice. It’s 128 days since my last ride in the UK.”
In response, the BHA said in a statement: "Prior to returning from a suspension of this nature, jockeys may be required to complete a course of testing, alongside other requirements, in order for BHA's Chief Medical Adviser to confirm their safe return to racing.
"As of 12 March Mr Murphy is only part-way through his reinstatement testing, and he is therefore unable to ride at this point."
