Prince William celebrates his 40th birthday on Tuesday.
It marks a significant milestone for the future king, who is rapidly stamping his authority on the British royal family by plotting a course between tradition and modernity.
This week, he spoke of his desire to end homelessness in the UK after taking to the streets of London to sell The Big Issue.
The Duke of Cambridge joined forces with the social enterprise, which offers a revenue stream to homeless people across the UK, to sell copies of the magazine in the capital before writing an article setting out why it is a cause so close to his heart.
“I was 11 when I first visited a homeless shelter with my mother, who in her own inimitable style was determined to shine a light on an overlooked, misunderstood problem,” he wrote.
"I refuse to believe that homelessness is an irrevocable fact of life. It is an issue that can be solved, but that requires a continued focus and comprehensive support network.”
UK's Prince William celebrates his 40th birthday
The prince also shared a photograph on Father's Day of him laughing with his children during a family holiday in Jordan, yet another shift from the more formal images in which the royals are usually feature.
Prince William was pictured with his arms around Prince George, 8, and Princess Charlotte, 7, while Prince Louis, 4, sat on his shoulders.
The Duke of Cambridge also released new photos to mark his 40th birthday.
The pictures were taken earlier this month when he took to the streets of London to sell the Big Issue.
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Prince William sells the Big Issue - in pictures
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His wife Kate celebrated her 40th birthday in January and only Queen Elizabeth II is held in higher regard by the public than the close-knit couple, who are often held up as a model for the future of the monarchy.
According to pollsters YouGov, the prince is the most popular royal behind the queen, with a 66 per cent approval rating, followed by Kate on 60 per cent.
"It's a very important milestone for him because, with his father Prince Charles, he's stepping up to support the queen, and also to continue to create an identity as the future king," royal commentator Richard Fitzwilliams told AFP this week.
Such is Prince William's popularity that many want him to leapfrog his father and become king when his grandmother dies.
Since the 96-year-old sovereign began winding down her duties due to ill health and old age last year, second-in-line to the throne Prince William has been an increasingly influential figure.
The prince accompanied his father when the queen missed the State Opening of Parliament in May and he has also weighed in on important family decisions.
He was reportedly a prime mover in blocking the participation of Prince Andrew, his scandal-hit uncle, in the traditional Order of the Garter ceremony this month.
The Duke and Duchess of Cambridge are "the monarchy's future, that was underlined at the balcony appearance at the end of the jubilee" celebrations to mark the queen's record-breaking 70-year reign, Mr Fitzwilliams said.
The duke's time has been devoted to the family since giving up his role as a helicopter ambulance pilot in 2017.
Significantly, he is expected to move out of London's Kensington Palace and into a four-bedroom cottage, described as modest, on the grounds of the queen's Windsor Castle home west of London where she spends most of her time.
The move will allow him to be closer to the queen and strengthen his role in the family's inner sanctum, commentators say.
While he is becoming more committed to his royal duties, he is also determined to provide a normal life for his three children.
This is partly driven by his own childhood, which was marked by the trauma of his parents' public separation and divorce, and the death of his mother Princess Diana in a car accident in 1997 when he was only 15.
His mother's influence is evident in his more modern outlook, which led to him breaking with the traditions of royal fathers, getting his hands dirty with nappy changing and helping the older two children with homework.
The prince is also keen to carry on his mother's legacy of helping society's most vulnerable members, hence The Big Issue connection.
Prince William was "moulded" by the "traditional queen" during Sunday lunches at Windsor when he was studying at the elite Eton College nearby, said Marc Roche, author of several books on the British monarchy.
But "among William's assets, there is … perhaps due to the influence of his mother, a great sensitivity to diversity and the environment", Roche said.
The prince has revealed little about what goes on behind palace walls but defended the family after his brother Prince Harry and wife Meghan gave a shock interview on US television, in which they accused royals of being racist.
He vehemently denied the allegations and by all accounts, relations with his brother are strained.
But Prince William recognises the need to modernise the age-old institution to enable it to survive after Elizabeth II and fend off the forces of republicanism that are stronger among younger generations.
This need was emphasised during a recent tour of the Caribbean in March, which was criticised for being a throwback to colonial times.
"One of the things that William is particularly concerned with is the image of the monarchy and how it moves forward," Mr Fitzwilliams said.
