Shanaze Reade, the British Olympian and two-time world BMX champion, has injured her right shoulder in a training accident, and has been ruled out of competing for four months.
The 20-year-old was due to make her mountain bike competition debut riding four-cross at this weekend's UCI World Cup round in Fort William, Scotland.
Reade fell off her bike and landed on the shoulder she injured two years ago. She's likely to need keyhole surgery.
"I'm very disappointed that I can't race at the BMX World's this year," she said. "I really wanted to get out there and defend my title. I'm also disappointed that I can't make my mountain bike debut in the four-cross at the World Cup.
"My focus now is on recovery and getting back into training as soon as I can, so I can begin my preparations for London 2012."