A woman who had been reported as missing is in a critical condition in hospital after she was involved in a collision with a police van which was trying to find her.
The woman, in her 50s, had been reported missing over fears for her own safety.
The marked police van was responding to the 999 call about the same woman.
The incident happened at 10.20pm on Thursday, July 27 on the B1135 Wymondham Road in Hethel, around eight miles south-west of Norwich.
Two ambulance crews and an ambulance officer was called to the scene at 10.25pm and treated her at the scene for complex serious head injuries and leg injuries.
She was taken to Addenbrooke's Hospital by the East Anglian Air Ambulance (EAAA), where she remains in a critical condition.
The helicopter was piloted by Prince William, who was working his last shift before leaving the life-saving charity.
A Norfolk Police spokesman said: 'Officers were in the area responding to a 999 call relating to concerns for the safety of a woman who had been reported missing.
'The road was closed while initial scene investigations were carried out and remains closed this morning.
'In accordance with normal procedure, Norfolk Police has referred the matter to the Independent Police Complaints Commission (IPCC).'
An EAAA spokesman said: 'The EAAA crew, which last night included William, attended an incident south of Norwich which involved a road traffic collision-related injury.
'A patient was treated at scene for severe head injuries then treated at Addenbrooke's Hospital.'
The road remained shut for nearly 17 hours before reopening just after 3pm.
The air ambulance returned to its Cambridge base at 1.30am, at the end of Prince William's final shift.
The Duke announced in January that he would be ending his career with EAAA after spending more than two years flying medical crews to emergencies.
An IPCC spokesman said they had not ruled out interviewing him as part of their investigation.
The spokesman said: 'When conducting investigations, the IPCC often works closely with the emergency services as well as other partner organisations to obtain relevant evidence.
It is therefore entirely possible in this investigation that witness statements may be required from employees working with the emergency services outside of Norfolk Police but thus far there has been no indication that this will be necessary.'
He joined EAAA as a pilot in March 2015. After completing an initial period of job-specific training involving simulator, aircraft and in-flight skills, he began piloting his first operational missions in July 2015.
Throughout his service, William has been based at Cambridge Airport, as part of a team including specialist doctors, critical care paramedics and pilots providing emergency medical services across Bedfordshire, Cambridgeshire, Norfolk and Suffolk.
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