The 16-year-old horse, who had been in wonderful form in four one-day events this year, with two wins, a second and a third, somersaulted over a rail which formed the second element of fence 15.
King William sustained minor cuts and bruising. His rider, who flew home yesterday for further treatment, has a straight fracture to her left wrist and a compound fracture to her left ankle.
King had not planned to compete at Badminton next month with her two top horses, Star Appeal and King Solomon III, who have both recovered from injuries sustained last year. She is still hoping to ride one or both of them at Burghley in September.
At the Rolex Kentucky Three-Day Event, Blyth Tait, the world and Olympic champion from New Zealand, saw his hopes of collecting a Grand Slam prize of nearly pounds 160,000 disappear by a frustratingly tiny margin when he rode Aspyring into second place. He finished just two points behind America's Karen O'Connor on Prince Panache.
But for the new rules which came into effect this year, Tait would have added a Kentucky victory to the one achieved at Burghley last year. A third victory at Badminton next month would have allowed him to claim the big prize.
Tait has been one of the more vociferous critics of the new rules, which were introduced to simplify the scoring. They have the effect of making the show jumping less influential, as was proved in Kentucky. Under the old formula, Tait's clear show jumping round would have left him 0.8 of a penalty ahead of O'Connor after her single error.
Results, Digest, page 23