Fans of the royal family know that Prince William is currently second in line to the British throne due to being the son of Prince Charles and grandson of Queen Elizabeth II, who is the current monarch. This positions Prince George as third, right behind the Duke of Cambridge. However, Express reports that there is a rule that could cause the two members of the family to lose their positioning.

The rule in question, which is laid out in the laws of succession, says that the order is not only determined by descent, but also by "Parliamentary statute."

"The Act laid down that only Protestant descendants of Princess Sophia - the Electress of Hanover and granddaughter of James I - are eligible to succeed. Subsequent Acts have confirmed this," reads the law, which is clearly stated on the website for the royal family.

This means that if either Prince William or Prince George stray from the Church of England that they would lose their spot in line. It also states that "a Roman Catholic is specifically excluded from succession to the throne" and the "Sovereign must also promise to uphold the Protestant succession."

READKate Middleton, Duchess Of Cambridge Might Not Become The Queen After All

Additionally, anyone who is expected to assume the role must also "swear to preserve the established Church of England and the established Church of Scotland." However, members of the royal family are still allowed to both marry Roman Catholics and retain their spot in line as 2013's Succession of the Crown Act ended the previous disqualification, but it comes with certain limitations.

When speaking with Express, Richard Fitzwilliams, a royal historian, said, "Prince George, Princess Charlotte and Prince Louis could marry a Catholic but their children would have to be brought up in the Church of England if they were to retain their place in the line of succession."

At this time, it seems as though the Duke and Duchess of Cambridge feel confident that Prince George will one day take the throne as they are said to already be prepping him at age six for his future role. 

"William talks to George about what being a royal means," an insider told Us Weekly, adding, "He does so in a very simple manner because he doesn't want his son to feel overwhelmed at such a young age."

Elaborating, the source said that the Duke drew pictures and used metaphors "as if he's reading from a storybook" in order to help George understand what's to come for him. 

Prince William and Prince George Prince William and Prince George arrive at the Lindo Wing after Catherine, Duchess of Cambridge gave birth to a baby girl at St Mary's Hospital on May 2, 2015 in London. Photo: Getty Images/Chris Jackson