Champion stayer Big Orange has been ruled out for the season because of injury.
Trainer Michael Bell says the Ascot Gold Cup winner, who has contested two Melbourne Cups, has damaged a suspensory.
"It's a body-blow for all concerned," Bell told Press Association Sport.
"The horse is 100 per cent sound and giving off all the right signs. But he had a little bit of heat in his leg tonight and though he jogged up sound on a hard surface, there is a small hole in his off-fore suspensory.
"It's a blow as he's our flag-bearer, but the horse is happy and healthy and the vet is cautiously optimistic that it should heal in time."
Big Orange defeated Order Of St George by a short head in a dramatic finish to the 2017 Ascot Gold Cup.
He was last seen finishing down the field in the Dubai Gold Cup at Meydan in March and was preparing to make a return to the track in the Henry II Stakes at Sandown on Thursday week.
Big Orange finished fifth in the 2015 Melbourne Cup and 10th the following year.