Britain's Prince Harry will turn 40 in California on Sunday, thousands of miles away from his royal roots, with no sign that distance is dimming interest in King Charles' younger son.

With his Netflix documentaries, his charity work, his legal battles with the British tabloid press and his rift with the rest of the royals, Harry is rarely out of the spotlight.

The prince will mark his 40th birthday with a private celebration at his Montecito home in southern California, where he lives with his American wife Meghan, 43, and their two children.

The milestone has prompted a slew of media stories and speculation back in Britain about what his future career might hold and whether he might return to the active royal duties.

In one lengthy article, the Sunday Times quoted unnamed friends saying Harry was at an unhappy crossroads in his life with one former adviser quoted as saying: "All he does is spend time looking back."

But sources close to the prince said Harry was looking to the future with excitement and a list of projects, including another Netflix special, this one about the elite world of professional polo.

Later this month he will attend events in New York for Travalyst, his initiative to make the travel industry more sustainable, and for The Diana Award, the charity set up in honour of his late mother Princess Diana.