The panel will meet again at a date to be decided to hear mitigation from Henderson's legal team before issuing a punishment.
In 2006, trainer David Flood was warned off for two years after being found guilty of four charges relating to the unauthorised administration of a painkiller to a horse before a Newmarket race.
Meanwhile, James Eustace has warned that War Artist will need the run as he makes his seasonal reappearance at Newcastle on Saturday. The six-year-old has not been seen out since finishing three-quarters of a length behind Marchand D'Or in the Group 1 July Cup at Newmarket last season.
The former South African sprint star is poised to run in the Chipchase Stakes as a stepping stone to a crack at King's Stand hero Scenic Blast, Golden Jubilee winner Art Connoisseur and Queen Anne Stakes victor Paco Boy in the July Cup.
But Newmarket trainer Eustace sounded a note of caution. He said: "We were trying to get him ready for Royal Ascot but it was always going to be a rush, so we decided against it.
"He's had a very long time off because of minor injuries. I am quite happy with War Artist, though he will come on a ton."
