As King Charles III continues to receive treatment for his cancer diagnosis, and with Princess Kate Middleton also battling cancer, the prospect of Prince William and Kate ascending the throne has reportedly left the couple feeling "intense anxiety," according to a royal expert.

Tina Brown, former editor of Vanity Fair, wrote in a recent article that "News of Charles' cancer has put William and Catherine in frightening proximity to ascending the throne," adding that "The prospect of it, I am told, is causing them intense anxiety," as reported by the Mirror.

The revelation comes as the royal family undergoes a major reshuffle in the wake of Charles' diagnosis, which was made public in February. The 75-year-old monarch has been forced to reduce his public-facing duties and undertake less of his royal agenda as he focuses on his treatment. In March, Kate Middleton also announced that she is in the midst of her own battle with cancer.

As the heir to the throne, Prince William, 41, is expected to become king of the United Kingdom once his father steps down or passes away. Kate Middleton, 42, will then be crowned queen and take on a significantly increased number of royal engagements.

In the absence of two senior members of the Firm, Queen Camilla, Princess Anne, Prince Edward, and his wife Sophie, the Duchess of Edinburgh, have each taken on a more active role to support the monarchy. However, the succession talks surrounding William and Kate have reportedly left the couple feeling uneasy about the immense responsibility that awaits them.

Tom Sykes, royal correspondent with Daily Beast, noted that "Some believe, however, that if Charles and Kate were to be permanently side-lined, forced into greatly reduced roles for the longer term by ongoing health challenges, a Prince William regency would look desperately short-handed. Officials are as busy as ever. There is no question of decisions being taken by anyone other than the king."

Despite the concerns surrounding his health, King Charles is said to be "raring to go" and is reportedly "supercharging" plans for a two-week Australia tour with Camilla later this year. Royal sources told the Sun that the monarch is "over the moon" that his cancer treatment has started well and that he is "feeling extremely positive after tests meant he could attend the Easter Sunday service and spend time meeting the public, which he has missed."

Charles' former butler, Grant Harrold, also revealed that the "workaholic" king has become "frustrated" over the current face of the monarchy, suggesting that he is eager to return to his full duties as soon as possible.

The news of William and Kate's anxiety over their future roles comes at a time when the royal family is facing unprecedented challenges, both in terms of the health of its senior members and the ongoing tensions with Prince Harry and Meghan Markle, who stepped down as working royals in 2020.