Germany's Pit Beirer of the KTM team was seriously injured in an accident during the fifth round of Motocross Grand Prix of Bulgaria near Sevlievo, some 220 kilometers (137 miles) east of Sofia.
Beirer fell over the largest jump on the course and lay motionless. He was immediately transported by a helicopter to the University Hospital in Lozenets.
Doctors say that his current condition is unstable and he can not be transporte to Germany now.
Pit Beirer's wife has been notified about the accident and is expected to arrive in Bulgaria on Monday. She has already given her consent that all necessary medical interventions are carried out in Bulgaria.
L&M Yamaha's Stefan Everts won the Grand Prix of Bulgaria Sunday in Sevlievo, eight seconds ahead of KTM's Joel Smets.
Brian Jorgensen with Honda ranked third, followed by World Champion Mickael Pichon.