Kate Middleton Becomes Princess of Wales After Princess Diana Following Queen Elizabeth II's Death

The title change comes after Queen Elizabeth II's death on Thursday.

Kate Middleton is now the Princess of Wales. Following Queen Elizabeth II's death on Thursday, King Charles III officially named his son and daughter-in-law, Prince William and Kate, the Prince and Princess of Wales, titles he once held with his late first wife, Princess Diana.

"Today I am proud to create him Prince of Wales, Tywysog Cymru, the country whose title I've been so proud to bear," King Charles said during his pre-recorded message from Buckingham Palace on Friday. "With Catherine beside him, our new Prince and Princess of Wales will, I know, continue to inspire and lead our national conversations, helping to bring the marginal to the center ground where vital help can be given."

Kate and William's new titles are reflected on both their Instagram and Twitter accounts.

"This was not automatic," royal expert Katie Nicholl told ET of William and Kate's new titles. "It was upon the wishes of King Charles. Of course those titles are hugely significant. King Charles was the longest serving Prince of Wales this country has ever had."

Meanwhile, the title of Princess of Wales "needs no introduction," Nicholl said, because of Diana's past success in the role. Kate's late mother-in-law, who is also mom to Prince Harry, became the Princess of Wales when she married the now-king in 1981.

"I'm told by Palace sources that Catherine is very well aware of the title that she has now inherited, but she will continue to do what she has done as the Duchess of Cambridge, I believe, carving her own niche within the royal family, pressing ahead in the very humble and modest way that she has done," Nicholl told ET. "Everyone will compare her to the last Princess of Wales, they were sizable shoes to fill. I suspect that Catherine's success as the Princess of Wales will be making that title her own."

For updates on Queen Elizabeth's death, visit ET's ongoing coverage here.