The owner of a champion dressage horse that had to be destroyed after being given cortico-steroid injections won damages of £350,000 on Monday.
French rider Michel Assouline used to compete with Annastasia
Jane McGarel-Groves, 54, of Ludgershall in Bucks, sued vets Erik Grandiere and Philip Glyn over Annastasia's death.
A High Court judge said the owner was not warned of the slight risk of laminitis arising from the treatment.
Mr Justice Forbes said 85% of liability lay with the French vet Mr Grandiere and 15% with Mr Glyn, of Sussex.
Solicitor Mrs McGarel-Groves said 14-year-old Annastasia - or Anna - was her "horse of a lifetime".
Mrs McGarel-Groves told the judge: "Anna was not only the most talented horse that I have ever owned (or am ever likely to own) she was also a kind and gentle mare."
Mr Glyn, of Priors Farm Equine Veterinary Surgery, in Forest Row East, became responsible for Anna in 1995 when she became based at the High Meadows training establishment in Sussex.
In March 1999, when the horse showed some stiffness in her left hock after competing with the French team Mrs McGarel-Groves agreed Anna should have the injections on the basis Mr Glyn would attend.
The judge agreed that had Mrs McGarel-Groves been warned that cortico-steroids could cause the potentially fatal hoof problem laminitis however, she would not have agreed to the treatment.
When the horse's performance dipped in May 2001, Mrs McGarel-Groves agreed to a repeat of the 1999 treatment as long as Mr Glyn was in attendance.
The judge said it was clear that at High Meadows Mr Grandiere, the new French team vet, administered two types of cortico-steroid to Anna by injecting one into her back and the other into each hock.
He said it was common ground that there was no clinical justification for the treatment and Mr Grandiere was therefore in breach of the duty of care he owed to Mrs McGarel-Groves and was negligent.
In his view, Mr Glyn was also in breach of duty by his "extraordinary" failure to make any inquiry as to the types of cortico-steroids to be used or their dosages.