Officers were called just after 10pm on Halloween night (October 31) to reports of a crash between a pedestrian and a car on Southend Lane, Bellingham.
A 65-year-old woman is in life-threatening condition.
No arrests have been made, and enquiries continue.
Officers are now appealing for any witnesses or anyone who might have information to contact the police.
Detective Inspector Paul Jackson, of the Met’s Serious Collision Investigation Unit, said: “If you were on Southend Lane or the local area at the time of the collision, please look back to see if you remember anything.
“We are keen to hear from anyone who may have witnessed this incident or who may have dashcam footage.
“If you can help police, please urgently call the Serious Collision Investigation Unit witness line number 02079608044 quoting 9111/31NOV25.”
