The popular seven-year-old stayer has damaged a leg which will keep him out of the 2018 Flat campaign.
Bell said: “It’s a body-blow for all concerned.
“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 renewal of the Ascot Gold Cup last season.
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.