Zara Phillip retired from competing at the Badminton Horse Trials after making a mistake on the cross-country course.
After a good start during yesterday's dressage stage, Phillips - who was riding High Kingdom - missed the second of two brush jumps in the Lake and pulled up.
It means she can no longer take part in tomorrow's show jumping stage.
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In an exclusive interview with ITV News, 100 metre world champion Justin Gatlin also said that booing at the World Championships "did hurt".
The 26-year-old had just given birth to her third child and was being transferred to a different floor when the tragedy happened.