Detectives are appealing for information and witnesses following a collision on the A406 on Monday night.
Police were called at 22:21hrs on Monday, 6 June, to reports of a collision between a car and pedestrian on the A406, Brentfield Road.
Officers, the London Ambulance Service and the London Ambulance Fire Brigade attended the scene and found an injured 54-year-old woman.
She was taken to a central London hospital where she remains with serious injuries
The driver of the vehicle stopped at the scene and is assisting detectives with their enquires. She has not been arrested.
Detectives from the Serious Collision Investigation Unit at Alperton are investigating.
At this early stage detectives believe the driver of a black cab may have witnessed the events leading up to the incident or the collision itself and are appealing for this driver to get in contact.
Anyone with information is asked to contact the witness appeal line at Alperton Traffic Garage on 020 8991 9555.