The B-sample taken from New Zealand eventer Jock Paget's Burghley-winning horse Clifton Promise has tested positive for the banned sedative reserpine.
The result was the same as the A-test last month, meaning Paget must now plead his case to the World Equestrian Federation (FEI) tribunal in the hope of avoiding a ban of up to two years.
The reigning Badminton and Burghley champion told Fairfax Media last month he was "absolutely dumbfounded" over his horse testing positive for the banned substance after his triumph at the four-star Burghley International Horse Trials in England in September.
The Olympic team bronze medallist was stood down from competition, pending the test of the B-sample. The rider is considered the person responsible for the horse.
Equestrian Sports New Zealand (ESNZ) received notification from the FEI today that the B blood sample taken from Clifton Promise had tested positive.
The result was widely expected as there have been few cases where the B-sample produces a different result to the A-sample.
Paget is now required to submit written documentation to the FEI explaining the presence of reserpine in Clifton Promise's bloodstream.
He was confident of providing the FEI with a comprehensive explanation which would be considered at a hearing of the tribunal, an ESNZ statement said.
He has 21 days to do so.
"I will continue to work closely with my team to provide a full explanation to the FEI." Paget said.
ESNZ was notified of the positive test from the A-sample on October 14 and since then both the rider and horse have been provisionally suspended from FEI and national competitions.
In keeping with ESNZ's own clean sport policy and its support of the FEI's clean sport programme, Paget has been stood down from any official ESNZ activity during the legal process.
Paget has always proclaimed his innocence and is searching for answers as to why the banned substance was found in Promise's blood.
- Fairfax Media