The Duke of Cambridge is due to start on Monday with the East Anglian Air Ambulance having taken unpaid leave in April before Princess Charlotte was born on May 2, the BBC reported.
The royal family - Prince Charles, Kate Middleton, Prince George and Princess Charlotte - has been at their home on the Sandringham estate in Norfolk.
William, 32, second in line to succeed his grandmother, Queen Elizabeth II, after his father, Prince Charles, is understood to be in the closing stages of his pilot training with the ambulance team.
He had previously served in the Royal Air Force, most recently as a search and rescue pilot.
The charity East Anglian Air Ambulance, which has bases in Norwich and Cambridge, ordered a new 1.7 million pound helicopter four years ago. It will come into service in Cambridge in April and is set to be flown by Prince William.
The air ambulance attends emergencies across Bedfordshire, Cambridgeshire, Norfolk and Suffolk.