The lightly-raced but high-class filly Haskoy has been forced into retirement after straining a tendon.
Ralph Beckett's filly only managed four starts on the track since making a huge impression when winning by seven lengths on her debut at Wolverhampton last July.
The daughter of Golden Horn stayed on strongly to land the Listed Galtres Stakes at York on her second start before coming home to fill the runner-up in the St Leger at Doncaster later that season, albeit she was demoted to fourth for causing interference.
The Juddmonte-owned filly was not seen until making a narrow winning reappearance in the Al Rayyan Stakes at Newbury last month and looked all set for a tilt at the Ascot Gold Cup next Thursday.
Trainer Ralph Beckett tweeted: "Haskoy has strained a tendon, consequently she is being retired to the paddocks."
She was as short as 5/1 in places for the day three feature even, a market that now sees the Roger Varian-trained Eldar Eldarov and Andrew Balding's Coltrane both top the betting at a general 3/1.
