Henderson is facing the prospect of a fine or a ban
Leading trainer Nicky Henderson could face a ban from horse racing after one of the horses he trained for the Queen failed a drugs test.
Henderson denied administering an anti-bleeding drug to Moonlit Path when she came sixth in a novices' hurdle at Huntingdon in February.
But a British Horseracing Authority hearing on Monday found him guilty of breaching the rules of racing.
The panel will hear mitigation from Henderson before deciding on a penalty.
Any ban could run from anything from one month to five years, while a fine could reach up to £12,000.
Henderson had admitted to three of the four charges he faced, but denied any wrongdoing in relation to the British Horseracing Authority's Rule 200, which governs administrating or attempting to administrate a positive substance.
The 59-year-old said nothing as he left after the eight-hour hearing, but in a statement last month he said the drug - tranexamic acid - had been administered by a vet entirely in the horse's interests.
Henderson insisted that there had been no intention to enhance the horse's performance.
A BHA statement read: "With the breach of Rule 200 being of such a serious nature, they (the panel) decided to deliberate longer on a suitable penalty before reconvening on a suitable date."
Moonlit Path was making her racecourse debut at Huntingdon and has since run three times.
The race was won by another Henderson-trained horse, Ravello Bay.