seems to be a genuine mistake rather than any attempt to deceive, mad how it can happen though.
An investigation is to be carried out after the 'wrong horse' won a race at Great Yarmouth at odds of 50-1.
The two-year-old Mandarin Princess, trained by Charlie McBride, was declared winner of the race at 13:40 BST after beating favourite Fyre Cay.
But a scan afterwards identified the horse as three-year-old stablemate Millie's Kiss, who had been due to run in a later race.
Stewards referred the matter to the British Horseracing Authority (BHA).
The result officially stands for betting purposes, but some bookmakers have said they will pay out on the horses finishing first and second.
The BHA described the incident as "unprecedented" since a microchipping identification system for horses was introduced in 1999.
It said: "The scan identified the horse to be Millie's Kiss, the trainer's other runner in race four.
"They (the stewards) 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they referred the matter to the head office of the British Horseracing Authority and ordered Millie's Kiss to be withdrawn from race four."
Mandarin Princess v Millie's Kiss
Mandarin Princess: Born 16 February 2015. No previous races
Millie's Kiss: Born 22 March 2014. Ten previous races, including four at Yarmouth. One win, at Lingfield.
Millie's Kiss had been due to run for Newmarket trainer McBride in the 15:15 race.
Stipendiary steward Tony McGlone told At The Races all horses are scanned before they are brought into their stables at the course.
"These two horses, trained by Charlie McBride, were given a box each," he said.
The responsibility lies with the trainer to present and run the correct horse in the race.
"Mr McBride went over to the weighing room to collect the saddle and was slightly delayed collecting it. He rushed over.
"The stable girl had taken the horse out of the stables and put it in the saddling boxes. Mr McBride put the saddle on, the horse ran, it won.
"We then sent the horse for routine testing as normal and the veterinary officer scanned the horse and found it to be the incorrect horse."
'Genuine mistake'
John Egan, who rode the 'winner', said he was surprised the horse was not disqualified or the race declared void.
"At the end of the day, we're all human. Mistakes happen. I feel most sorry for Charlie," he said.
John Mear, a member of the Four Winds Racing Partnership that own Millie's Kiss, said they were "shocked and upset" for the trainer.
"It's nothing untoward. He's just made a genuine mistake. He must be absolutely heartbroken," he said.
McBride was unavailable for comment.
What the BHA says
"The issue had not been established until after the result had been made official. After the weighed in has been declared on the racecourse, the result cannot be amended by the stewards.
"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. We sympathise with the betting operators and betting public who have potentially been affected by this incident."
Analysis
BBC racing correspondent Cornelius Lysaght
Though this may read like something out of a Dick Francis novel, sources at the British Horseracing Authority are indicating this looks more like a c**k-up than anything else.
Both horses are bay coloured fillies, and the BHA will seek to find out how they were confused by Charlie McBride and/or his staff; whatever, McBride is looking at a substantial fine.
All horses are microchipped so they can be checked on arrival at the track, but, after this, a second ID check, probably in the paddock before the race, is likely to be brought in.
