The collision happened at approximately 4.15pm on Thursday, 8 July on the slip road off the A316 leading to Nallhead Road.
A black Ford Fiesta, travelling westbound, was in collision with a cyclist on the slip road.
London Ambulance Service attended and the cyclist, a 25-year-old man, was taken to hospital for treatment – he remains there in a critical condition. His next of kin has been informed.
The driver of the car stopped at the scene and is assisting police with their enquiries. There have been no arrests.
Detectives from the Serious Collision Investigation Unit are asking anyone who was in the area and witnessed this incident, or any road users who captured events on dashcam, to contact them on 0208 543 5157 or dial 101 and quote CAD5463/8July. You can also tweet information to @MetCC.