Zara Phillips and Tsunami II at Bramham in 2007.
© Jan Milne
The 10-year-old mare Tsunami II fell on the landing side of the 15th jump, a straightforward hedge, at the three-star event on Saturday.
Tsunami II (MayHill x El Nino) was transported to a veterinary centre for further examinations but it was discovered the horse had a fracture in the second cervical vertebrae in her neck. She was euthanised on the agreement of the veterinarians involved.
The anglo-european mare was owned by Melanie Duff and Zara Phillips.
Phillips, 27, was taken to a local hospital, where it was found her collarbone was broken. She was given strong painkillers by medical staff and had her arm put in a sling. She made her way back to Britain last night.
She will be out of action for the rest of the season, adding to her disappointment at having to withdraw from Britain's Olympic team when Toytown was injured.
A friend said Tsunami II was one of Phillips's favorite horses and she was very upset about the loss.
Pau was the first four-star event for Tsunami II, who had 208 British horse trials points. She had been placed third in an event at Burnham Market in September. Phillips took over the ride in March 2005 from Camilla Stewart-Wood.
Pau spokesman Pascal Sayous said: "The horse fell after jumping over a small hurdle of laurel. It was the easiest obstacle on the course. Zara broke her collarbone. Her shoulder is in a very bad way. We're waiting for a full medical report.
"The horse broke a vertebrae in the neck. This is terrible for a horse, it cannot recover from this. The vets decided to put the horse down.
"It was a very ordinary accident, but it's not common to see such a terrible outcome. In the 20 years that I've been doing this job, we've only had to put down two horses, including this one."