A horse owned by Queen Elizabeth has tested positive for a prohibited substance, the British Horseracing Authority (BHA) has said.
Trainer Nicky Henderson has been charged by the BHA in connection with a sample from the mare Moonlit Path taken after a race at Huntingdon in February when she finished sixth of 11 in a novices' hurdle.
The sample contained tranexamic acid, a legitimate drug commonly marketed as Cyklokapron and used on humans as well as animals to prevent or treat excessive bleeding.
It is allowed by horse racing authorities in some parts of the United States but not in Britain where horses must be drug-free on race day.
Henderson, 58, one of the country's leading trainers, won the Champion Hurdle at Cheltenham in March for the fourth time.
He issued a statement, saying: "Unfortunately Moonlit Path tested positive for a prohibited substance after finishing sixth on her debut at Huntingdon.
"The substance concerned was administered by my vet entirely in the interests of the horse's welfare, which is always paramount. There was no intention to enhance performance and it is worth pointing out that we won the race in question with the odds-on Ravello Bay.
"We have co-operated fully with the BHA throughout their investigation and I would prefer to make no further comment before the hearing."
The BHA said on their website that Henderson had been charged with a breach of the rules in that he "allowed or caused to be administered, or connived at the administration of, tranexamic acid."
This had been done "either with the intention of affecting her racing performance or in the knowledge that her racing performance could be affected by such."
An inquiry is due to be held in June on a date to be confirmed.
Moonlit Path is one of 41 horses owned by Queen Elizabeth, a lifelong racing enthusiast. They include Barbers Shop who ran seventh in the Cheltenham Gold Cup in March.
Moonlit Path has run twice since Huntingdon, finishing fourth of 13 both times.