Champion New Zealand horseman Jock Paget faces a lengthy ban and the loss of his Burghley eventing title after the B sample from his horse Clifton Promise returned a positive test for a banned substance.
Clifton Promise was tested after Burghley in September and last month returned a positive reading for Reserpine in his blood. It is a sedative often used to help horses recover from long term injury or illness.
With the B sample checked and the same substance found, Paget now has three weeks to prepare a dossier of evidence before appearing in front of an International Equestrian Federation tribunal.
The whole process could take about six weeks.
Equestrian Sports New Zealand said it had received notification from the International Equestrian Paget said that while the B sample result was disappointing, it was expected as the blood in the B sample was taken at the same time as the A sample.
"I will continue to work closely with my team to provide a full explanation to the FEI,'' he said.
ESNZ was notified of the positive test from the A sample on October 14 and since then both the rider and horse had been provisionally suspended from FEI and national competitions.
Paget has been stood down from any official ESNZ activity while the legal process is underway.
ESNZ chief executive Jim Ellis said Paget was understanding and fully supportive of ESNZ's position as the FEI judicial process took its course.
"ESNZ is confident that he will provide a full and persuasive explanation to the FEI.''