The prince is understood to be the first member of the Royal Family in the direct line of succession to sign such a contract.
Contrary to speculative reports, William’s main home base with his wife the Duchess of Cambridge and son Prince George will be Kensington Palace, which has just had a £4.5 million taxpayer-funded refurbishment.
After his training he will work from both Cambridge and Norwich Airports, flying day and night shifts, from spring 2015. A Kensington Palace spokesman said: “His Royal Highness is hugely excited and motivated about this opportunity.
“He sees this as public service in its truest sense, helping people in the most difficult of circumstances on the front line. He sees it as a privilege to be able to serve in this way.”
William will also continue his work with charities and with the Royal Foundation of The Duke and Duchess of Cambridge and Prince Harry.”
A source said: “It is the perfect opportunity for him. He can combine royal duty with spending time with his family and his passion for flying.
“The Queen, the Prince of Wales and his wife the Duchess of Cambridge fully support his decision.”
He will start as a co-pilot but, after a period of training, will be qualified to fly as a helicopter commander. He will fly doctors and paramedics to emergency situations across the region.
The new role will build on William’s operational experience in the Royal Air Force Search and Rescue Force.
Between January 2010 and September 2013 he was based at RAF Valley on Anglesey where, as Flight Lieutenant Wales, he served as a fully operational Search and Rescue pilot on a Sea King helicopter. He qualified as a helicopter captain in June 2012.
During his time with the RAF he undertook more than 150 search and rescue operations, completing 1,300 flying hours.
He left Anglesey in September 2013, after more than seven-and-a-half years of full-time military service.
