The royal family has revealed the reason behind Queen Camilla's prolonged illness, prompting the cancellation of a significant public event.

Queen Camilla's ongoing chest infection continues to affect her royal duties.

Her Majesty, who reduced her duties for the Qatar state visit this week as she recovers, informed guests at the Buckingham Palace festivities that her recent illness has evolved into pneumonia, according to People.

The Queen, who did not attend the traditional kickoff at Horse Guards Parade in London Tuesday morning, was driven to the palace in the afternoon for the luncheon welcoming Sheikh Tamim bin Hamad Al Thani, the amir of Qatar, and his wife, Her Highness Sheikha Jawaher bint Hamad bin Suhaim Al Thani, according to the outlet.

The amir of Qatar was welcomed by King Charles, Kate Middleton, and Prince William.

Buckingham Palace did not respond to Planet Chronicle Digital's request for comment.

On Nov. 5, Camilla's events for that week were canceled as Her Majesty was advised to take a "short period of rest."

The BBC reported that a statement from the palace said that Her Majesty The Queen is currently unwell with a chest infection and has been advised by her doctors to take a short period of rest.

King Charles III and Middleton both received cancer diagnoses in February and March, respectively.

Kate Middleton underwent planned abdominal surgery and was diagnosed with a non-cancerous condition. However, further testing revealed cancer was present. Her medical team advised her to begin chemotherapy.

"The Princess of Wales stated in a prerecorded message in March that the news came as a huge shock and that she and William have been working hard to process and manage it privately for the sake of their family. It has taken time for her to recover from major surgery and start her treatment, but the most important thing has been explaining the situation to George, Charlotte, and Louis in a way that is appropriate for their ages and reassuring them that she will be okay."

In April, His Majesty resumed his royal duties, while Middleton, who announced she was cancer-free in September, has gradually started her comeback.

Planet Chronicle Digital's Lauryn Overhultz contributed to this post.