Timbera, the clear ante-post favourite for Saturday's Grand National, has been ruled out of Aintree after coming down with a lung infection.
The Dessie Hughes trained Irish National winner had been quoted as short as 8-1 with Paddy Power after receiving substantial support from punters.
"He's got a lung infection and won't run," said trainer Dessie Hughes.
With Gold Cup-winning pilot Jim Culloty due to ride Timbera, and carrying only 10st 5lb, confidence had been high at Osbourne Lodge Stables in Kildare.
Hughes will now try to get Timbera ready for a repeat bid to win the big race at Fairyhouse on Easter Monday.
"I don't know at this stage how long he will be out for, but we've got two weeks," he added.
Irish-trained horses will make up almost a quarter of the field in the £600,000 Aintree extravaganza, but Timbera's absence will be a major loss in efforts to land the race for the fourth time in six years.
Ted Walsh's Rince Ri was also pulled out of the National after scoping badly on Saturday