Kicking King, winner of the 2005 Cheltenham Gold Cup, has been ruled out of the blue-riband event on Friday.
Tom Taaffe's 10-year-old made an encouraging return when second to Nickname at Gowran Park in January, but disappointed when only third on his second run this year.
Connections were still keen to have a crack at the Cheltenham Gold Cup but they have been forced to bypass the Festival after Kicking King failed to sparkle in a piece of work this morning.
Owner Conor Clarkson said: "Unfortunately after a work-out this this morning Tom Taaffe wasn't happy, and neither was Barry Geraghty, with his work.
"We've therefore no option but to say he will not be running in the Gold Cup. Obviously there just something a little amiss at the moment, his legs are fine but the quality would not be up to the standard of the Gold Cup. He's just not firing on all cylinders.
Clarkson insists Kicking King will not be retired after this setback and will instead be primed for the Punchestown Gold Cup in April - a race he won three years ago.
"This has nothing to do with his retirement, it is just the quality of his work," he added.
"His work was reasonable but Tom is an astute trainer and Barry is a top jockey. The quality of his work was just not up to the quality that he'd been producing in the last couple of months. There may be a lingering, low-grade virus there and he'll have a nice little break now.
"If he shows sparkle in his work in the next couple of weeks we'd be delighted to go to Punchestown. The Punchestown Gold Cup is now the target."
Kicking King, also a winner of the King George VI Chase in 2004 and 2005, was as low as 20/1 to regain his Gold Cup crown on March 14.
The Big G