Police, ambulance crews and the air ambulance attended the A49 in south Shropshire, between the A489 (near Craven Arms) and the B4371 (Church Stretton) earlier this afternoon.
Highways England said: "Road users are advised to allow extra time for their journeys and may wish to consider alternative routes."
Jordan Eggington from West Midlands Ambulance Service said: "We were called at 12:54pm to reports of a road traffic collision involving a car and motorcyclist on the A49 from Marsh Cottage Junction in Felhampton.
“On arrival, the motorcyclist, a man, was found to be in a critical condition after coming off his bike in the collision. Crews carried out advanced trauma care to the patient at the scene and continued treatment on route to Queen Elizabeth Hospital.
“A man and woman also involved in the collision were treated for minor injuries.”