Haskoy, a leading contender for the Gold Cup at Royal Ascot, has been retired after straining a tendon.
Trained by Ralph Beckett, Haskoy did not make her racecourse bow until last July, when she ran out a seven-length winner on the all-weather at Wolverhampton.
The daughter of Golden Horn made rapid strides, winning a Listed race at York before coming home second in the St Leger, although the filly was demoted to fourth after causing interference in the closing stages.
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Haskoy was as short as 5-1 for the Gold Cup after a game Group Three victory on her Newbury return last month, but the setback means she will now head to the paddocks rather than the Royal meeting.
Beckett tweeted: "Haskoy has strained a tendon, consequently she is being retired to the paddocks."
Desert Crown set to miss Royal Ascot
Desert Crown will miss the Prince of Wales's Stakes at Royal Ascot, Sir Michael Stoute has confirmed.
Last year's Derby winner was on course for a clash with Adayar, Luxembourg and stablemate Bay Bridge in the 10-furlong Group One contest on June 21, having produced a scintillating piece of work under Ryan Moore on Wednesday.
However, after a routine canter under Ted Durcan on Saturday morning, he was found to be lame.
A market drifter on the exchanges on Sunday morning, asked if all was well with the colt, Stoute told the PA news agency: "No. He's not running, no. He's a non-runner."
Frankie Dettori was booked to ride the son of Nathaniel at Royal Ascot, replacing Richard Kingscote, but Bruce Raymond, racing manager to owner Saeed Suhail, added: "I knew they weren't happy with him yesterday morning.
"He worked well on Wednesday. He doesn't usually work like a rocket. His general work practice is to go steady early and to quicken."