The hearing into Jock Paget's failed doping test from last year's Burghley Horse trial has started in London with the New Zealand equestrian avoiding the media by entering the meeting from a back entrance.
Paget was suspended from competition after his horse, Clifton Promise tested positive for the banned substance reserpine at Burghley.
Paget and Clifton Promise won the event, but where then stripped of the title which then went to another New Zealander Andrew Nicholson.
Paget's former mentor Kevin McNab is also at the hearing after his horse also failed a drugs test.
Both horses are owned by Frances Stead of Clifton Eventers
The hearing will decide if Paget and McNab are at fault for the banned substance being in their horse's system.
If found guilty both could face a minimum two-year-ban from the sport.
The tribunal is expected to last a couple of days, but a verdict may not be reached for a further four to six weeks.