The children of Prince Harry and Meghan Markle are now entitled to the title of His Highness upon the Queen's death.
This means that sussex's children now have the titles Prince Archie and Lilybert Mountbatten-Princess Windsor.
The change was made in accordance with the rules laid down by King George V in 1917.
As children of the monarch's sons, the grandsons of the newly named King Charles III, they now also have a title in the style of their highness if they wish to use it.
Megan previously claimed her son was stripped of the title of prince in an explosive interview with Oprah Winfrey.
She also accused Buckingham Palace of failing to protect Archie, denying him 24/7 safety.
Asked if archie's being called a prince meant to her, Megan said she had no attachment to the "grandeur" of the official title.
But she said it was about "the idea that our son is insecure and the idea that the title of the first person of color in this family isn't named like other grandchildren."
This tradition stems from the patent certificate issued by Prince Harry's great-great-grandfather, George V, in 1917.
Debret's spokesman had previously told The Mirror: "In 1917, King George V decreed that only the monarch's grandchildren had the right to use the style of a prince or princess and his royal highness.
"The only exception to this order is that the eldest son of the Prince of Wales is also entitled to use the style of His Highness and the Prince.
This means that only Prince George, as the great-grandson of the monarch who was directly inherited to the throne, is eligible to become a prince.
He was the eldest son of the eldest son of the Prince of Wales.
The Queen stepped in before George was born, issuing a patent certificate to ensure that all children at Cambridge University had the proper title.
That didn't extend to Archie and his sister Lilybert.
However, with Charles now king, they are now grandchildren of the monarch and can be called his royal highness and prince or princess.
Queen Elizabeth II died at the age of 96, surrounded by her family at her beloved Scottish estate of Balmoral.
The news was announced on Thursday, in the Platinum Jubilee, 70 years after the monarch ascended to the throne, making her the longest-serving monarch.
Her eldest son and heir, Prince Charles, is now king and Camilla is queen.
The UK is now in a period of formal mourning, and her funeral will take place in two weeks.
Buckingham Palace announced at 6.30pm: "The Queen died peacefully this afternoon in Balmoral. The King and Queen will remain in Balmoral tonight and will return to London tomorrow.