Harry and Megan — who, Page Six is told, insist on being called "Ma'am" and "Sir" by staffers — have also used their HRH titles when traveling, sources said.

Meghan Markle recently used the HRH title in personal correspondence, including a gift basket sent to podcaster Jamie Kern Lima.

Despite Meghan Markle and Prince Harry's decision to step back from their royal roles, the British monarch has limited power when it comes to stripping royal titles.