Coneygree, who won the Cheltenham Gold Cup in March, has been sidelined until next year with an injury.
The horse - the first novice to win the Gold Cup for over 40 years - will miss the festive meetings.
Some reports have suggested the eight-year-old is out for the season.
Sara Bradstock, wife of trainer Mark, told the Racing Post, "Coneygree won't run in the King George (at Kempton on Boxing Day) having sustained a new injury."
She added: "He's being assessed by vets this morning."