Prince William's 40th birthday this week 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.

William's wife Catherine celebrated her landmark 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, William, whose birthday is on Tuesday, is the most popular royal behind the queen, with a 66-percent approval rating, followed by Kate on 60 percent.

"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.  

Such is William's popularity that many even 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 William has been an increasingly influential figure.

The Duke of Cambridge, as he is officially known, accompanied his father when the queen missed the State Opening of Parliament in May and has also weighed in on important family decisions.

He was reportedly a prime mover in blocking the participation of Prince Andrew, his sex scandal-hit uncle, in the traditional Order of the Garter ceremony this month.   

"They (William and Kate) 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, said Fitzwilliams.

Central role

William's time has been devoted to the family since giving up his role as a helicopter ambulance pilot in 2017.

Significantly, he is reportedly planning 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 get closer to the queen and strengthen his role in the family's inner sanctum, commentators said.

While he is becoming more committed to his royal duties, he is also determined to provide a normal life for his three children -- Prince George, aged eight, Princess Charlotte, seven, and four-year-old Prince Louis.

This is partly driven by his own childhood, which was marked by the trauma of his parents' very public separation and divorce, and the death of his mother Diana in a car accident in 1997 when he was only 15.  

His mother's example is evident in his more modern outlook, which saw him break with the traditions of royal fathers and get his hands dirty with nappy changing, and help the older two with homework.    

The tall, blond, partially bald prince is also keen to carry on his mother's legacy of helping society's most vulnerable members.

He was spotted in London earlier this month selling The Big Issue, the UK's homeless magazine, and has spoken out frequently on other issues such as the environment and mental health.

Diana's legacy 

William was "moulded" by the "traditional" queen during Sunday lunches with her at Windsor when he was studying at the elite Eton College nearby, Marc Roche, author of several books on the monarchy, told AFP.

But "among William's assets, there is... perhaps due to the influence of his mother, a great sensitivity to diversity and the environment", he added.

The prince has revealed little about what goes on behind palace walls but defended the family after his brother Harry and wife Meghan gave a shock interview on US television, in which they accused members of being racist.

He vehemently denied the allegations and by all accounts, relations with Harry are strained.

But 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," said Fitzwilliams.