Prince William is leaving Britain's armed forces to take up royal and charity duties, beginning with a key role in a new global wildlife conservation partnership, Kensington Palace said on Thursday.
The second-in-line to the British throne, who became a father in July, completed his last shift as a Royal Air Force search and rescue helicopter pilot on Tuesday after seven years in the military.
The 31-year-old Duke of Cambridge is in a "transitional" year, sources have said, and is considering options for his "public service" with a formal announcement to be made about his decision within the next 12 months.
But he said on Thursday that he would start by bringing together seven top conservation organisations in a group called "United for Wildlife", as part of which he has filmed a video with former footballer David Beckham.
The prince said that the new group, of which he will be president, would "provide the impetus for a renewed commitment and action to protect endangered species and habitats for future generations".
Speaking of the danger of extinction, William added: "We must work together to prevent this catastrophe and allow our children the opportunity to experience wildlife in its many beautiful and varied forms."
William's change of career comes just two months after his wife Kate gave birth to a baby boy, Prince George of Cambridge.
Kate made a sparkling return into the limelight on Thursday as she accompanied her husband to an awards ceremony recognising outstanding conservationists, dressed in an eye-catching silver gown.
Kensington Palace said in a statement: "His Royal Highness The Duke of Cambridge is to leave operational service in the Armed Forces.
"He completes his tour with the Royal Air Force Search and Rescue Force at RAF Valley, Anglesey, after more than seven-and-a-half years of full-time military service.
"He will continue to support the work of the Queen and the Royal Family through a programme of official engagements, both at home and overseas, with the Duchess of Cambridge.
"The Duke will work closely over the next 12 months with the Royal Foundation of The Duke and Duchess of Cambridge and Prince Harry. He will expand his work in the field of conservation, particularly in respect of endangered species."
William will continue to work with charities dealing with children and military veterans, the palace said.
The wildlife partnership brings together the foundation with Conservation International, Fauna & Flora International, the International Union for Conservation of Nature, The Nature Conservancy, Wildlife Conservation Society, WWF-UK and the Zoological Society of London.
William's late mother Diana was a frequent visitor to Africa where many endangered species have their habitats, while his father Prince Charles is a keen conservationist.
Prince William said earlier this week in a documentary screened by CNN and the British network ITV that the arrival of his son had increased the depth of his feelings towards endangered animals.
"The wildlife is incredibly vulnerable and I feel a real protective instinct, more so now that I am a father, which is why I get emotional about it... you want to stand up for what is very vulnerable and needs protecting," he said.
"Elephants, rhinos and many other animals that are persecuted don't have a voice."
William, Kate and George are expected to move into refurbished apartments at Kensington Palace in London within weeks.
The couple have previously spoken of how much they have enjoyed living near the RAF base in North Wales where they were largely left alone by the press, apart from the occasional photograph of the former Kate Middleton shopping in the local supermarket.
William's brother Harry remains in the military.