Police are appealing for witnesses and information following a collision in Hillingdon.
Officers were called by London Ambulance Service at 17:32hrs on Monday, 25 February to the A40, Hillingdon Circus, following a road traffic collision involving a car and a motorcycle.
The rider of the motorcycle, a man aged in his 50s, was taken to hospital by London's Air Ambulance. His condition is being treated as life-threatening.
There have been no arrests. The driver of the car is assisting police with their enquiries.
Road closures were put in place while the scene was dealt with.
Any witnesses to the collision, especially anyone with dashcam footage, is asked to call police on 0208 246 9820. Alternatively call 101 and quote CAD 5641/25Feb.