Seabass has been withdrawn from the Crabbie's Grand National at Aintree after suffering a serious injury at Down Royal on Monday.
Trained by Ted Walsh and ridden by his daughter, Katie, Seabass was third at Aintree in 2012 and went off the 11-2 favourite under last year only to finish 13th.
However, the 11-year-old finished last in his warm-up in the Daily Mirror Chase, after which he was found to have sustained ligament damage.
Pat Glynn, of owners the Gunners Syndicate, said: "He'll be out of action for at least six months and maybe eight (months).
"We were looking forward to him at Aintree again as he was coming back to form.
"He suffered ligament damage mid-way through the race. We thought he wasn't 100 per cent, and when he got home he was very lame. He had a scan this morning and they found ligament damage.
"He's had tendon trouble before and made a full recovery. If he's OK we might put him back into training, but we're not sure about that at the moment.
"He'd been working very well at home and we had big hopes for him. He's been a very good servant for us."