Kensington Palace said Prince William will join the East Anglian Air Ambulance flying both day and night shifts.
It will become the Duke of Cambridge’s main job, but his rota will take into account any duties he will continue to undertake on behalf of the Queen.
A spokesman said Prince William was “hugely excited and motivated” by the role.
“The duke sees this as a true form of public service, helping people in their most difficult times,” he said.
“He regards his work with the RAF search and rescue force as having been an exceptional privilege and the duke wanted to make his own contribution to the outstanding work of the air ambulance service.”
The charity’s chief executive, Patrick Peal, said: “We’re delighted His Highness has decided to fly with us.
“We are confident this will help raise the profile of the charity and other air ambulance charities in the region and across the UK.”
The charity said Prince William would be based at Cambridge Airport and fly missions in Cambridgeshire, Norfolk, Suffolk and Bedfordshire.
He will be paid a salary which he will donate in full to the charity, Kensington Palace said.
Cambridge Airport is well placed between the Duke and Duchess of Cambridge’s new home at Anmer Hall in Norfolk and their apartment at Kensington Palace.
There had been speculation surrounding the move since Prince William, who is qualified to be a captain or pilot of a Sea King helicopter, ended his active service as an RAF search and rescue pilot last September.
During his service he carried out more than 150 missions and completed more than 1,300 flying hours.
His main duties will involve flying an EC145 T2 aircraft and working alongside medics to respond to emergencies ranging from road accidents to heart attacks.
In order to gain this license, Prince William must complete five months of training followed by 14 exams and a flight test.
Initially Prince William will be employed as a co-pilot but, after a period of training he will be qualified as a helicopter commander.
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