Stewards have opened an investigation after scratching the Simon Morrish-trained Show A Star from a race at Flemington on suspicion the horse had received a race day treatment.

Show A Star was taken out of Saturday's VRC-CRV Cup Tour Trophy (2000m) just over 20 minutes before the start of the race.

"It was withdrawn by order of stewards, acting under Australian rule 178AB, because the stewards reasonably suspect the horse has had an injection to its near-side neck either today or yesterday," Racing Victoria acting chief steward Robert Cram said.

"It's the result of a report from the compliance team stewards who visited the stable complex at 8am this morning."

It is against the rules for a horse to receive treatment within one clear day of racing.

Show A Star had been among the favourites for Saturday's race.