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.
For more on the Badminton Horse Trial visit the ITV News West Country (E) website.
Zara Phillips put herself firmly in contention at the Mitsubishi Motors Badminton Horse Trials today with a quality dressage performance on her London 2012 horse High Kingdom.
Phillips, the Queen's granddaughter, posted a dressage score of 44.3 penalties that kept her firmly in touch with the leading group midway day two.
Braving the cold at the Cheltenham Festival the coats have stayed on and lots of fur has been on show, but despite the weather the hats have been no less impressive.
Styles this year seem to favour smaller hats with plenty of feathers making an appearance.
How does Ladies' Day compare to last year?
Gallery: Ladies dress to impress at Cheltenham Festival 2012.
Zara Phillips is putting her equestrian skills to a new use - by perfecting the Gangnam Style dance.
The dance, which involves bounding around as though straddling a horse, shot to worldwide fame after becoming a hit on You Tube.
Phillips, the granddaughter of the Queen, will be joined by other figures from the Olympic eventing team for the performance in aid of Children In Need.
Zara Phillips has told Daybreak that she is hungry for more success after becoming the first British royal to win an Olympic medal.
Joined by the rest of the Team GB eventing team, she told Daybreak: "We'll take it one year at the time, in our sport it's better to do just yearly goals but being at one Olympics make you hungry for the next one."
Zara Phillips delivered a good performance on her Olympics cross-country debut at Greenwich Park to keep Great Britain firmly in medal condition.
The Queen's granddaughter was watched from the main arena stands by the Duke and Duchess of Cambridge, Prince Harry and the Duchess of Cornwall.
The 31-year-old former world champion produced a clear round inside the time, completing a course that claimed its fair share of casualties.
Seven of the first 27 combinations fell and were eliminated, but Phillips kept control all the way around a testing track that was very slippery in places.
Five combinations were clear inside the time as the competition entered its second half - Phillips, Wilson, Germany's Ingrid Klimke, Sweden's Sara Algotsson Ostholt and Australian Christopher Burton.
Phillips finished on her dressage score of 46.10.
Zara Phillips been chosen alongside William Fox-Pitt, Mary King, Piggy French and Tina Cook for London 2012.
She clinched her spot in the team following a third-placed finish at the Bramham International CIC three-star class after posting a personal best dressage score, and then jumping clear in the showjumping and cross-country phases.
Speaking about her Olympic call-up, Zara Philips said: "High Kingdom is a pretty cool, very relaxed kind of guy, I was really happy with him at Bramham as he’d obviously grown up and is improving all the time.
"He’s pretty ponylike, a nippy little jumper and easy to manoeuvre so hopefully it will suit himwell in Greenwich.
"High Kingdom is owned by Trevor Hemmings who’s been one ofmy earliest supporters – he’s owned a lot of my horses and has been sosupportive, I couldn’t do it without him.”
Zara Phillips has been picked to represent Great Britain at this summer's Olympic Games in London.
The Queen's granddaughter described her selection to the eventing team as "awesome".
Phillips, the 2006 world champion, is following in the footsteps of her parents, who both rode in the Olympic Games for the country.
Her mother, the Princess Royal, competed at the 1976 Montreal Games, while her father Captain Mark Phillips was a team gold medallist at Munich in 1972 and then won silver in Seoul 16 years later.
Phillips said: "It's awesome to be given this opportunity.
"I am really excited and can't wait to kick on and get him there. Hopefully, we will make it this time."
The eventing star will ride High Kingdom at Greenwich Park next month.
The Daily Telegraph is reporting that the Queen’s grand-daughter Zara Phillips has been selected for one of five places on the British eventing team for London 2012:
Zara Phillips selected for Olympic team
From @pkelso on Twitter:Her mother, Princess Anne, represented Britain at the 1972 games in the same event and her father Mark Phillips won a team gold in the same year.