English racing has been left embarrassed by the discovery the Charlie McBride-trained Mandarin Princess who was declared winner of a two-year-old race at Yarmouth, was in fact her elder stablemate Millie's Kiss.

The mistake was only discovered after the 50-1 outsider had overturned the 4-6 favourite Fyre Cay on Thursday (Friday AEST).

Because correct weight had been announced, the result was allowed to stand - for the time being.

Millie's Kiss, a three-year-old with plenty of racecourse experience, was due to run in a race later in the day.

"The stewards held an inquiry to consider the circumstances surrounding the identification of the winner, Mandarin Princess, trained by Philip (Charlie) McBride, which was presented at the sampling unit for routine testing," the stewards' report said.

"The scan identified the horse to be Millie's Kiss, the trainer's other runner in race four.

"They interviewed the trainer, the stable groom, the veterinary officer and the equine welfare integrity officer responsible for the sampling unit.

"Having heard their evidence they referred the matter to the head office of the British Horseracing Authority and ordered Millie's Kiss to be withdrawn from race four."

Most bookmakers paid out on both horses with the British Horseracing Authority to launch an investigation.

"Since we introduced the micro-chipping identification system an incident such as this is, as far as we are aware, unprecedented," a BHA statement said.

"The issue had not been established until after the result had been made official.

"The responsibility lies with the trainer to present and run the correct horse in the race.

"Having said that, and while we have not seen an incident of this nature in recent times, we will of course determine what steps need to be put in place to prevent it from happening again."