Michael Whitaker is out of the Beijing Olympics showjumping competition after his horse Portofino 63 became lame.
The team veterinarian has carried out an extensive investigation but as yet has been unable to target the source of the problem.
The 14-year-old bay mare, owned by Gillespie Equestrian, was sound during pre-export quarantine in the UK and on arrival in Hong Kong but has developed a problem in the last 48 hours.
The British Olympic Association and the British Olympic Equestrian Team decided it was in the horse's best interests to withdraw her before the horse inspection.
The reserve horse, Russel, ridden by Nick Skelton, now comes into the squad of four.