Kate Middleton, who recently celebrated her 40th birthday, is expected to receive a royal title when Prince Charles becomes a monarch if Queen Elizabeth II decides to abdicate or passes away. However, it would be a difficult moment for Prince William as it reminds him of his mother, Princess Diana.

According to Express UK, unlike the Duchess of Cambridge, the late Diana never made it to her 40th birthday as she passed away at the age of 36 in a tragic car accident.

Following her death, her "Princess of Wales" title remains vacant.

The outlet explained that Camilla Parker Bowles could take the title whenever she wants as she's married to Prince Charles, but she personally decided not to use it out of respect for the late princess.

When Prince Charles becomes United Kingdom's monarch, he has to bestow Prince William the title of "Prince of Wales."

A source spoke to the Telegraph saying, "it's hard to see it not happening," and they mentioned that it would be a "bittersweet moment" for Prince William as it could become a "full circle moment."

"it will also be a poignant reminder of what we all lost," the insider continues, referring to Princess Diana's death.

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Kate Middleton Pays Tribute to Princess Diana, Queen Elizabeth II for her 40th Birthday Photoshoot

To embark on a new milestone in her life as she enters her fourth decade, Kensington Palace released three portraits of Kate Middleton over the weekend.

According to Today, the Duchess of Cambridge wore Alexander McQueen dresses, the same designer who designed her wedding gown in 2011.

Middleton donned a vibrant red dress with a statement sleeve and pockets in the first photo. Her choice of accessories stood out as she wore a pair of diamond earrings.

The Court Jeweller reported that the accessories were loaned to the Duchess by Queen Elizabeth II.

Middleton also paid tribute to Princess Diana by wearing a pair of Collingwood pearl and diamond earrings that once belonged to her mother-in-law.

The earrings were not the only accessories she wore for the shoot as she also showcased the Princess of Wales' sapphire engagement ring, which Middleton wore in 2010 after getting engaged with Prince William.

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