The King was notified of Harry and Meghan’s intention to use the title for their children, a royal source has confirmed.
A statement came after the couple confirmed their little girl was christened on Friday as Princess Lilibet Diana.
It said: “The appropriate conversations have taken place ahead of today’s news (of Lilibet’s christening).”
Harry and Meghan's christening took place privately at the Sussexes’ home in California with around 20 to 30 people in attendance for the celebration.
A spokesperson for Harry and Meghan said: "I can confirm that Princess Lilibet Diana was christened on Friday, March 3 by the Archbishop of Los Angeles, the Rev John Taylor."
Lilibet, one, and her older brother Archie, three, weren't given HRH titles at birth due to their position in the line of succession.
Harry and Meghan are understood to be keen to not deny their children, both Lilibet and Archie, their birth-right.
It is the first time Lili has been publicly referred to as Princess.
She became entitled to become a princess when her grandfather King Charles acceded to the throne.
It is understood Lili's title of princess and Archie's title as a prince will be used in formal settings, but not in everyday conversational use by the couple.
However, they will allow them the chance to decide for themselves when older whether to drop or keep using the titles.
When Archie and Lilibet were born, they were great-grandchildren of the Monarch, which meant they were eligible for HRH titles.
It also applies for Prince George, Princess Charlotte and Prince Louis, however the Queen stepped in to change the rules.
Prince George needs to be a HRH as he will one day be king.
But as Harry is less senior than his brother, it didn't apply to his children.
When Charles became King in September last year, Archie and Lilibet became the grandchildren of a Monarch which changed the roles.
Buckingham Palace said the royal website “will now be updated in due course” to reflect the titles.
King Charles was reportedly undecided over whether his two youngest grandchildren can use their titles of prince and princess and their HRH stylings.
His Highness reportedly waited until after the release of Prince Harry's autobiography 'Spare' in January before making a decision on his grandchildren's royal titles.
