Police were called to a crash between a cyclist and a car in Ditches Lane on Monday (November 3) at 4.20pm.
The cyclist, 46, was taken to hospital where he remains in a life-threatening condition.
The driver of the car, a red Hyundai i20, stopped at the scene and is assisting officers with their investigation.
No arrests have been made at this stage and enquiries are ongoing.
Detective Sergeant Jim Hinchcliffe, from the Serious Investigation Collision Unit, said: “We are looking for anyone with information to get in touch.
“If you were in the Ditches Lane area at the time of the collision and may have been a witness, have dashcam footage, or if you have any other information please get in touch by calling 020 8543 5157 and quoting CAD 5284/03NOV.”
