The Duke of Cambridge has begun his final shift for the East Anglian Air Ambulance (EAAA).
His last duty as a paid pilot for the EAAA is the night shift from its base at Cambridge Airport.
Writing in the Eastern Daily Press (EDP) Prince William said he had a "profound respect" for those who serve in the emergency services.
He is stepping down to take on more royal duties on behalf of the Queen and Duke of Edinburgh.
A former RAF search and rescue pilot, the duke is part of a team including doctors and paramedics providing emergency medical cover across Bedfordshire, Cambridgeshire, Norfolk and Suffolk, and to Essex and Hertfordshire at night.
Earlier this year, he said: "It has been a huge privilege to fly with the East Anglian Air Ambulance.
"Following on from my time in the military, I have had experiences in this job I will carry with me for the rest of my life, and that will add a valuable perspective to my royal work for decades to come."
After two years in the role, he told the EDP: "I have met people from across the region who were in the most desperate of circumstances.
"As part of the team, I have been invited into people's homes to share moments of extreme emotion, from relief that we have given someone a fighting chance, to profound grief."
Speaking of the "incredibly skilled doctors and paramedics" he has worked with, the prince said: "These experiences have instilled in me a profound respect for the men and women who serve in our emergency services, which I hope to continue to champion even as I leave the profession.
"I am hugely grateful for having had this experience."
Pilot William Wales, as he is known at work, receives a salary for his job which he donates in full back to the EAAA charity.
While on duty, he works as part of a close-knit team of four, on a nine-and-a-half hour shift, attending the worst medical emergencies of the 2,000-plus calls per day the service receives.
By Peter Hunt, royal correspondent, BBC News
As a prince and as a future king, William has worn and will wear plenty of uniforms.
But the flight suit he'll hang up after his last shift has particular significance.
For two years it's been his "passport" to a life where, on merit, he helped people save lives.
This after all is a man who wishes that when he was younger he could have done more to protect his mother.
He has been exposed to the National Health Service in a way that no other senior royal has been or possibly ever will be.
It's an experience he is determined will shape his future.
The words the East Anglian Air Ambulance has used to describe the pilot prince are warm, not perfunctory.
They are losing someone they call "much-loved", "hard-working" and a "wonderful character".
William is losing something he's cherished - working in a team.
A lonelier destiny, which he's put on hold for so long, now beckons.
The air ambulance charity has attended patients injured by fires, horseback riding accidents, poisoning and road traffic accidents.
"There are some very sad, dark moments. We talk about it a lot, and that's the best way of dealing with some of these situations," Prince William revealed in September.
He pilots a H-145 helicopter, which has a maximum speed of 145 knots (170mph) and can be on the scene of an incident in East Anglia within a matter of minutes.
William has been an "integral part" of the service, Patrick Peal, chief executive of EAAA, said.
"He is not only a fantastic pilot, but a much-loved and valued member of the crew. He will be truly missed by everyone at EAAA.
"William... has been a true professional, delivering our doctors and critical care paramedics to patients under testing conditions," he added.
The Duke and Duchess of Cambridge, whose family home has been in Anmer, Norfolk, have taken up residence in Kensington Palace ahead of their eldest child, Prince George, starting school in September.
A statement issued in January by the palace, said the pair wanted to increase their official duties on behalf of the Queen and their charity work - which would mean more time in London.
"As I hang up my flight suit, I am proud to have served with such an incredible team of people, who save lives across the region every day," Prince William told the EDP.