Police are appealing for witnesses to a road traffic collision on the A413 near Stoke Mandeville.
A white BMW 5 Series and a purple and black Yamaha motorcycle were involved in the collision on the A413 Wendover Road at the junction with Quilters Way at 5.40pm on Tuesday (15/8).
The motorcycle collided with the side of the BMW as the BMW was making a turn in the road due to stationary traffic.
The motorcyclist, a man in his twenties, was taken to the John Radcliffe Hospital in Oxford with life-threatening injuries, where he remains in a critical condition.
No arrests have been made at this stage of the investigation.