Police were called at 8.25pm on Saturday, 31 July to reports of a collision involving a car and a motorcyclist in Maida Vale Road, near to the junction with Elgin Avenue.
Officers attended along with London Ambulance Service. The motorcyclist, a 28-year-old man, was taken to hospital where he remains in a life-threatening condition. His next of kin has been informed.
The driver of the car stopped at the scene and is assisting police with their enquiries.
No arrests have been made.
Detectives from the Serious Collision Investigation Unit would like to hear from anyone who was in the area at the time and witnessed the collision.
If you can help please call the appeal line on 020 8543 5157. Alternatively call 101 quoting CAD 6343/31Jul.