Irish rider Cian O'Connor was on course for a clear round until he appeared to be distracted by a member of the ground staff running across the arena during the event in Aachen which doubled as the Olympic qualifying contest.
Television pictures showed a staff member, wearing a yellow shirt, running across O'Connor's path and jumping into a flower bed. The Irishman's horse knocked down the next fence.
The incident cost Ireland a place at the Rio de Janeiro games as they finished one place below Spain and out of the Olympic qualifying spots. If O'Connor had gone clear, the positions would have been reversed.
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Ireland had already had an appeal against the result rejected by the International Equestrian Federation (FEI).
CAS did not give any reasons for its decision.
"The CAS Panel has issued only its decision today, without the grounds, which will follow in the coming weeks," CAS said in a statement.
(Writing by Brian Homewood, editing by Ed Osmond)
