Police were called at about 4:05am last Friday (15 July) to reports of a collision between two cars on Stockley Road at the junction with the M4 off-slip in West Drayton.
Police along with the London Ambulance Service and London Fire Brigade attended the scene.
A woman, who was a passenger in one of the cars, was taken to hospital by ambulance and continues to receive medical care.
The drivers of both cars stopped at the scene and are assisting police with the ongoing investigation. There have been no arrests.
Detectives from the Serious Collision Investigation Unit are investigating the circumstances of the collision. They are particularly keen to hear from any witnesses, or anyone who captured the collision on dashcam.
Any witnesses are asked to call 020 8246 9835 or via 101 quoting reference CAD 1072/15Jul.