Detectives from the Met's Roads and Transport Policing Command are appealing for witnesses following a collision in Hounslow.
Police were called by the London Ambulance Service at approximately 18:40hrs on Saturday, 7 January to reports of a motorist taken unwell on Worton Road at the junction with Farnell Road in Isleworth.
At this early stage of the investigation, it is believed that he collided with an oncoming light coloured, possibly silver, VW Golf before coming to a stop in a hedge.
The male driver, aged in his 70s, was taken to a west London hospital where he remains in a critical condition.
The driver of the VW Golf briefly stopped at the scene before driving away.
Detectives would like to speak with this driver, or anyone else who witnessed the collision.
Anyone with information is asked to contact the Serious Collision Investigation Unit on 020 8543 5157 or via 101 and @MetCC