Prince William flew a critically-injured woman to hospital during his final shift as an air ambulance helicopter pilot.
The woman, in her 50s, was hit by a police car near Hethel, home of Lotus Cars, near Norwich.
Officers were responding to a 999 call "relating to concerns for the safety" of the woman, who had been reported missing.
She was treated at the roadside for serious head injuries before being taken by the East Anglian Air Ambulance (EAAA) to Addenbrooke's Hospital in Cambridge.
She was last night said to be in a "life-threatening condition".
No one else was hurt in the late-night accident.
A mile-long stretch of the country road remained closed yesterday as the Independent Police Complaints Commission (IPCC) investigated.
"In accordance with normal procedure, Norfolk Police has referred the matter to the IPCC," said a Norfolk Police spokesman.
The air ambulance returned to its Cambridge base at 1.30am at the end of William's final shift.
He announced in January that he would be leaving the East Anglian Air Ambulance to spend more time on his royal duties.
He qualified as a pilot at Royal Air Force College Cranwell in Lincolnshire in 2008.
He then underwent further training to become a full-time helicopter pilot with the RAF Search and Rescue Force in 2009. His service with the British armed forces ended in 2013.
He joined the air ambulance in March 2015 and flew his first operational mission after three months' training.
Throughout his service, William has been based at Cambridge airport as part of a team providing emergency medical services across Bedfordshire, Cambridgeshire, Norfolk and Suffolk.
A statement issued by Kensington Palace earlier this year said William and his wife Kate wanted - as they had in previous years - to increase their official duties on behalf of the Queen, and their charity work, which would mean spending more time in London.