King Charles III has agreed to give Prince Harry and Meghan Markle's children the titles of prince and princess following his ascension to the British throne.

The Duke and Duchess of Sussex's two children, Archie, 3, and Lilibet, 1, were not given prince and princess titles at birth due to the interpretation of the letters patent, a royal expert told Fox News Digital.

The letters patent says grandchildren of the sovereign are entitled to the titles prince and princess. 

"So this was a big contention during the Oprah interview, because Meghan told Oprah Archie was entitled to be a prince and that the family had denied him. That was not right. That was incorrect," royal expert Shannon Felton Spence explained.

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"Archie was not yet the grandchild of the sovereign," she added. "He was the great-grandchild of the sovereign. So titles are made up there. The sovereign is the CEO with this made-up guidance memo, and they can interpret it however they want. And, at that time, that had been interpreted that Archie was not the grandchild of the sovereign."

Now that Charles has become king, Prince Harry and Markle's children are grandchildren of the sovereign. 

"I think that it's beautiful now that today is Harry's birthday, and the king has granted them the titles," Spence said. "It's a privilege of being the grandchild of the sovereign. This is great, and I think that it should show that all that was preventing it was a document."

Prince William and Kate Middleton's children received prince and princess titles upon birth, even though they weren't grandchildren of the sovereign, because they are directly in line for the throne, the royal expert said.

Spence also explained why Harry and Markle's children will likely not receive His/Her Royal Highness (HRH) titles at this point.

"Harry's always had one and Meghan got hers when they got married because they were working royals," Spence told Fox News Digital. "Their first full-time job was working royals. Using the HRH titles just enables additional privileges, like state-funded security. So that's why we no longer call Prince Harry, His Royal Highness, Prince Harry, the Duke of Sussex, or Her Royal Highness, Meghan, the Duchess of Sussex. They're not going to take them away, but they don't use them.

"So Archie and Lili won't get them right now because they're not working royals, and their parents don't use them," she continued. "That doesn't mean that they won't ever get them. It just means that, at this moment, they don't have HRH. It doesn't really make any difference to their lives in California at all."

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However, Harry and Markle continue to campaign for their children to have HRH titles, likely for security reasons. The Duke and Duchess of Sussex are angry over the lack of HRH titles, according to reports.

Prince Harry is in a legal battle with the British government over his family's lack of security in the United Kingdom. He and Markle were stripped of their security detail after stepping down from their royal roles.

The prince has previously pointed to his mother's death as a reason for needing the heightened security. Princess Diana died from injures she sustained in a car crash. The crash occurred while the driver attempted to flee from paparazzi.

"Diana lost her HRH upon divorce, and that has always been seen by many observers of the royal family as something that was quite cruel to take from her," Spence told Fox News Digital. "Particularly given that she was still raising the two princes."

"At the time of her death, she did not have HRH. Harry and William have always sort of felt, and they have said quite publicly, that the fact that she didn't have that extra security — particularly Harry. Harry brings it up quite a bit — contributed to the tragic death of his mother," she added. "And, part of it, he thinks was that she didn't have that state security. She was sort of on her own post divorce."

Outside of security reasons, it's unclear why Markle would want the HRH titles.

"They have said in the interview that she doesn't care about titles, that's not something that she is looking for. But their whole brand is based around being royal adjacent or being royal," Spence said of Markle. "And so, they use these titles, which they are entitled to use.

"There's a dissonance between what they say and then how hard they fight for them. They say, 'We don't want to be working members of the royal family,'" she added. "'The royal family has treated us very badly. Our mental health can't take living in the UK or doing that job and all of that is fair.' But then why fight so hard for the titles and put them so front and center of your brand?"

Just because Archie and Lilibet aren't being granted HRH titles now, doesn't mean they won't become working members of the royal family in the future.

"Archie and Lili could get the HRH titles at some point. It's not like King Charles is decreeing that they can never have them," the royal expert said. "They made it quite clear that when Harry and Meghan left, the door would be open to them to returning into working royal lives if they ever wanted.

"And, of course, if that happened, then it’s plausible Archie and Lili could get their HRH and become working members of the family."