Fame And Glory, who was among 13 horses confirmed for Thursday's Gold Cup at Royal Ascot on Friday, has been trimmed to 5-4 (from 6-4) by William Hill to retain his crown.
Victory for the star stayer, who could be joined by his Aidan O'Brien-trained stablemate Memphis Tennessee, is "easily" the worst result in Hill's ante-post Royal Ascot book.
"There has been plenty of confidence in Fame And Glory's Gold Cup double bid and he's easily the worst result in our Ascot ante-post book so far," said the firm's spokeswoman Lucy Rhodes.
Potential opposition includes Godolphin duo Colour Vision and Opinion Poll and Irish raider Saddler's Rock, who is trained by John Oxx.
The Gold Cup takes place 40 minutes after the Group 2 Ribblesdale Stakes for three-year-old fillies.
Shirocco Star, The Fugue and Vow - second, third and fourth to Was in the Oaks - were among 22 confirmed for the Group 2.