Jockey Jamie Moore has broken his leg in a fall at Towcester and will miss the Grand National on 11 April.
Moore was due to ride 33/1 shot Al Co, last season's Scottish National winner, at Aintree for trainer Peter Bowen.
He is the sixth leading rider to be ruled out of the 2015 National, joining the injured Mark Walsh, Davy Russell, Barry Geraghty and Jason Maguire, plus suspended Bryan Cooper.
"I'm not in any pain, just gutted to miss Aintree, that hurts," said Moore.
The rider's father, trainer Gary Moore, said: "He's broken his leg and I'm not sure how it happened. It was just one of those crazy things and I feel so sorry for him."
Jamie is the regular jockey on the 2014 Queen Mother Champion Chase winner Sire De Grugy.
His brother Josh will ride the horse should he run in the Melling Chase at Aintree's National meeting or the Celebration Chase at Sandown on 25 April.