Great Britain's Liam Phillips crashed out of the BMX World Championships in Birmingham, suffering a shoulder injury which could yet scupper his Olympic hopes.
The 23-year-old, from Burnham-on-Sea in Somerset, claimed silver in the time-trial on the opening day of elite competition, but his participation in Saturday's super cross qualifying lasted only to the first corner of the opening heat at the National Indoor Arena.
After a promising start, Phillips was among the leaders of the eight riders entering the first corner but tumbled to the dirt following the first jump and landed heavily over the handlebars.
Phillips, whose progress has been hampered by a series of injuries in recent years, was taken from the track on a stretcher and was assessed by British Cycling's medical staff.
The Olympic BMX competition takes place from August 8 and if Phillips is unfit to ride, the host nation place would likely be filled by an academy rider.
Shanaze Reade, seeking a fourth world title in the Olympic discipline, was untroubled in the first and second of her three qualifying heats as she sought a quarter-final place.
And as Phillips was being taken to hospital for an X-ray, Reade won her third qualifying heat to go through to the quarter-final.