Leading John Smith's Grand National contender Tidal Bay has suffered a "tiny stress fracture" and will not run at Aintree on 6 April.
Allotted top-weight of 11st 10lb in the Liverpool showpiece, the Paul Nicholls-trained 12-year-old has not run since winning the Lexus Chase at Leopardstown over Christmas.
He was forced to miss the Argento Chase at Cheltenham last month when reported to be lame, and Nicholls said in his Betfair blog: "As previously mentioned, he was slightly lame before the Argento and we couldn't get him ready in time for that race, or the Irish Hennessy.
"He was out cantering as usual on Monday and Tuesday, but on Wednesday we decided to step up his work.
"But his work rider came back up and said he didn't feel right.
"So we immediately called in Buffy (Shirley-Beavan) and, after discussions, we got him down to Newmarket today for an MRI scan.
"The news through from Buffy is that Tidal Bay has a tiny stress fracture of the lower cannon bone on his right hind leg. This is something that the X-rays we took before his problem in the Argento didn't pick up; only today's MRI did.
"He needs a month's box rest, so that rules out the National.
"As soon as I got the news, I immediately rang Mr (Graham) Wylie and we agreed to release this statement as soon as possible."