Police are investigating in the wake of the incident in Kentish Town in North West London. A witness said they heard “a loud bang” and saw a cyclist on the ground who was “in a lot of pain” after an accident involving the Labour leader.
The cyclist was taken to hospital with an injured left arm.
Sir Keir, 58, exchanged insurance details with the cyclist before heading away from the scene.
The former Director of Public Prosecutions was asked to report to a police station.
A Met Police spokesman said: “Police were alerted by LAS at around 12.20hrs on Sunday 25 October to a report of a road traffic collision between a cyclist and car in Grafton Road, NW5.
“The driver of the car had stopped at the scene and exchanged details with the cyclist but had left before officers arrived.
“The male cyclist received a minor injury to his arm and was taken to hospital by LAS as a precaution.
“Officers later attempted to contact the driver of the car and left a message advising him to report the matter to police.
“The driver of the car subsequently attended a north London police station. He was not arrested or interviewed under caution.