They used the title princess for their daughter, known as Lili for short, when she was christened in Los Angeles on Friday.
It was the first time Lilibet has been publicly referred to as a princess.
Why did Archie and Lilibet not previously have their royal titles?
Technically Harry and Meghan’s children became a prince and princess when the King acceded to the throne, but they have remained “master” and “miss” on the Buckingham Palace website for the last six months.
The palace has confirmed the site’s line of succession list would now be updated to reflect the change, after a spokesperson for the Sussexes publicly referred to Lilibet as a princess when announcing news of her christening.
Title rules set out by King George V in 1917 gave the children and grandchildren of a sovereign the automatic right to the HRH, prince and princess titles.
When Archie was born, he was the great-grandchild of a sovereign rather than a grandchild, so was not given the title.
However, when the Queen died and her son King Charles acceded to the throne, it meant Archie and Lilibet were entitled to be a prince and princess.
They are also entitled to use the HRH style – although Harry and Meghan retain their HRH styles, they no longer use them after leaving the working monarchy.
How will the titles be used?
It is understood the titles will be used in formal settings but not in everyday conversational use.
As such, the announcement of Lillibet’s baptism in California was the first opportunity to use her title since the death of the Queen.
Harry and Meghan are keen not to deny their children their birthright, but wish to allow them the chance to decide for themselves when they are older whether to drop or keep using the royal titles.