A boy, believed to be three, has been critically injured after being struck by a car, police said.
Officers have appealed for witnesses after the incident in Uxbridge Road, Hayes, on Sunday.
The driver of the car stopped and no arrests have been made.
In a tweet, Hillingdon Borough Police said: “We’re working at the scene of a collision at 7pm between a car and a child in Uxbridge Road #Hayes.
“The boy, aged around 3 years, is critically injured. Driver of the car stopped; not arrested.”
Anyone with information is asked to call police on 101.