The Cheltenham Gold Cup is the blue-riband event of the Cheltenham Festival, which begins on Tuesday.
It features the top National Hunt horses racing over a trip of three miles and two-and-a-half furlongs while tackling 22 fences.
Sizing John won the Cheltenham Gold Cup in 2017 but will not be back to defend his crown.
The eight-year-old was dramatically ruled out last week after suffering a hairline fracture.
Trainer Jessica Harrington was hoping Sizing John would become the first horse to defend the Cheltenham Gold Cup since Best Mate in 2003.
Sizing John’s jockey Robbie Power said: “We’re all very disappointed but don’t worry, he’ll be back.”
When is the Cheltenham Gold Cup?
The Cheltenham Gold Cup takes place on Friday, March 16.
It is scheduled to start at 3.30pm GMT.
How can I watch the Cheltenham Gold Cup?
ITV Racing will broadcast live coverage of the 2018 Cheltenham Festival on ITV – channel number 103 on Sky and Virgin Media.
Express Sport will also have the Cheltenham Festival covered, so check back each day for our live blogs.
Which horses are running in the Cheltenham Gold Cup?
American, Anibale Fly, Bachasson, Definitly Red, Djakadam, Double Shuffle, Edwulf, Killultagh Vic, Mala Beach, Might Bite, Minella Rocco, Native River, Our Duke, Outlander, Road To Respect, Saphir Du Rheu, Shantou Flyer, Tea For Two and Total Recall.
The final 48-hour declaration stage is 10.00am GMT on Wednesday, March 14.