King Charles' horseDesert Hero came third in the St Leger on Saturday.
According to the BBC, Desert Hero, owned by the King and Queen, stayed on for a third with the Royal couple watching on.
Bred by Queen Elizabeth II, who died a year ago, a win for Desert Hero would have been the first Royal victory in a Classic since Dunfermline won for King Charles III's mother in 1977, the publication reported.
Animal rights groups are expected to criticize King Charles and his wife for taking part in the horse races.
The race came months afterpeople belonging to Animal Rising took three lambs from King Charles's Sandringham Estate in Norfolk.
Police guarding the Epsom Derby made multiple arrests including of an activist from the same group who stormed the track during the blue-riband horse race.
In the hours ahead of the race, Surrey Police arrested 19 activists from the animal rights group in a pre-emptive operation around the Epsom racecourse, near London.
All were arrested on suspicion of conspiracy to commit public nuisance, the police force said.
Animal Rising had said before the race that it was undeterred both by the arrests and a court injunction obtained by the Jockey Club this week.
Three horses were destroyed after getting injured during the Grand National steeplechase festival -- proof, the group says, that racing is fatally dangerous.