Thistlecrack out of Cheltenham Gold Cup and rest of the season with injury
Thistlecrack will miss the rest of the season, including March's Cheltenham Gold Cup, with a stress fracture.
The 2016 King George VI Chase winner only returned to racing in December after 10 months out with a leg injury.
"We were going for the Gold Cup - we're gutted," assistant trainer Kim Tizzard told At the Races.
She added: "He was a little bit stiff behind after the King George and he's been off and had X-rays and bone scans - it's gutting.
"He ran a screamer. He jumped really well and we were getting excited that the old Thistlecrack was still there.
"Hopefully he'll have his box rest and come back next season.
"We're gutted for the team, the owners, everyone. We've still got a horse, at the end of the day."
The 2018 Cheltenham Gold Cup takes place on Friday, 16 March.
Thistlecrack missed the 2017 Gold Cup with a slight tendon tear, having started the season by winning his first four starts over fences.
Frank Keogh, BBC racing reporter
This is a real blow for jump racing fans, with one of the sport's most popular steeplechasers missing the blue riband event for the second successive year.
Thistlecrack has built up a big following, with that brilliant King George win providing his ninth victory on the trot.
It is often difficult for steeplechasers to return to their best after serious injuries, but plenty will hope this can be the exception.