Police were called at 4.49am on Saturday, 31 July to reports of a collision involving a car and a pedestrian on the A3, near to the junction with Rydal Gardens.
Officers attended along with London Ambulance Service. The pedestrian, an 82-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 helping officers with their enquiries. There have been no arrests.
Detectives from the Serious Collision Investigation Unit are carrying out enquiries to establish the circumstances of the incident and would like to hear from a number of other drivers who were in the area at the time.
If you can help, please call the Witness Appeal Line on 020 8543 5157. Alternatively call 101 quoting CAD 1210/31Jul.
