A man died in a collision with a car in Hillingdon after he was apparently trying to chase after his dog.
London Ambulance Service (LAS) attended the scene and treated the 35-year-old male pedestrian, who was suffering from critical injuries.
He died at the scene shortly after.
Officers believe he was trying to catch his dog, who is described as a German Shepherd or Alsation, moments before the collision.
Next of kin have been in formed and a post-mortem examination will be held in due course.
Formal identification will also awaits.
The driver of the car, a silver Peugeot 307, stopped at the scene.
He was not arrested and is assisting police with their enquiries.
Police are now appealing for information from anyone who saw the collision as officers from the Roads and Transport Policing Command investigate.
Detective Sergeant Stephen Pidgeon, who leads the investigation, said: "We understand the deceased was trying to catch his dog, described as a German Shepherd or Alsatian, in the moments prior to the collision.
"As a result of this the deceased ended up in the road where he was struck by the car.
"If anyone remembers seeing this scenario or the collision itself, please contact police."
Anyone who witnessed the collision is asked to contact police via the witness appeal line on 020 8991 9555.