An expert revealed that King Charles advised Prince William to avoid the mistakes he made in his marriage.

King Charles gave Prince William a caution about his marriage.

Ingrid Seward, editor-in-chief of Majesty magazine and author of "My Mother and I," stated that "His father" said, "Look, William, there’s nothing more important than your family."

""I just did duty, duty, duty. I neglected my marriage, and look what happened," Seward claimed he said."

And there are lessons to be learned from the king's past.

In 1970, Charles met Camilla Parker Bowles at a polo match. Despite becoming close, their relationship was disrupted by his eight months of naval duty. During his absence, Camilla married her longtime boyfriend, Andrew Parker Bowles, in 1973.

In 1981, Lady Diana Spencer married Charles, but their union was problematic and subject to intense tabloid scrutiny.

In 1995, Camilla and Andrew divorced, while Charles admitted to an affair with his former love, and in 1996, Diana and Charles' divorce was finalized, a year before the princess's tragic death at age 36.

"[His father] said, ‘Look, William, there’s nothing more important than your family. Don’t make the mistakes that I made. I just did duty, duty, duty. I neglected my marriage and look what happened.’" — Ingrid Seward, author of ‘My Mother and I’

In 2023, Charles and Camilla were crowned king and queen, having married in 2005.

In 2001, William and Kate Middleton met while studying at St. Andrews University. The following year, they lived together with friends. In 2003, they moved into their own home, a year before their relationship became public.

In 2007, the couple split up after dating for nearly five years, but later reunited. William proposed to Kate in 2010 and the couple got married in 2011.

The British throne's heir is William, who has three children with his wife, the Princess of Wales: Prince George, Princess Charlotte, and Prince Louis.

Seward stated that Charles was a strong advocate for the union and urged William to prioritize duty when Kate experienced a significant health crisis.

In January, Kensington Palace declared that Kate had undergone a "planned abdominal surgery." To be with Kate and support their family, William put off official royal duties.

In March, the 42-year-old princess announced that cancer had been present after she became the subject of online conspiracy theories surrounding her health.

William took time off to support his wife after Kate's announcement.

Kate, the princess, announced in September that she was cancer-free. She described the past nine months as difficult and expressed "relief" at completing her course of treatment, alongside her husband and their children.

"Seward stated that she believes Prince William has received excellent support from him. She mentioned that William publicly acknowledged the year as the most challenging of his life, and it was indeed difficult. Given that William also had to reassure their three young children that their mother would be okay, it is safe to assume that the situation was even more challenging. Children often worry when they see their parents unwell, especially for an extended period."

"Seward stated, "It's been tough, but he has been a huge support. In return, his father has supported him.""

Kate expressed excitement about returning to work in her video announcement, but also stated that she intended to approach each day individually. Seward observed that Kate had been managing well.

"Seward stated that she does not typically perform evening engagements, but she has a carol service at Westminster Abbey on Christmas Eve that will be recorded and televised."

An emotional letter was sent by Kate to each of the 1,600 guests attending the Westminster Abbey gathering on Dec. 6, as reported by People Magazine.

She stated that the holiday season is one of her most cherished periods of the year.

""When we pause and distance ourselves from the stresses of daily life, we create room to live with compassion, empathy, and forgiveness, which are the core values of the Christmas season," she stated, according to the source."

The story of Christmas serves as a reminder of the significance of empathy, both in giving and receiving, and how crucial it is to rely on one another despite our differences.

Kate emphasized the importance of turning to love rather than fear. She encouraged us to show love to ourselves and others, love that is empathetic, kind, understanding, forgiving, and brings joy and hope.

"The greatest gift we can receive is this love. Love is the light that shines bright, even in our darkest times. We all have something we can offer one another, such as gentle words, a receptive ear, an arm around an exhausted shoulder, or silently being by someone's side."

Kate emphasized that her carol service was a heartfelt celebration of everyone present, reminding them that at Christmas and throughout the year, they must shine for each other.

"In both times of happiness and sorrow, we serve as each other's source of illumination."

WATCH: KING CHARLES GAVE THIS WARNING TO PRINCE WILLIAM ABOUT MARRIAGE: AUTHOR

It's likely that Kate and William, 42, will be accompanied by other members of the royal family and their three children, as they have done in the past.

Seward stated that Catherine possesses a significant warmth towards others, which is a trait that Princess Diana also had. He added that Catherine is completely sincere and believes that helping others brings joy to everyone involved.

Seward stated, "I believe she will focus greatly on that in the future." He added, "She'll simply do what she can. I think that when you're sick, as she has been - she didn't just have cancer, but also an abdominal operation - it must have been very frightening."

"I am confident that she is adhering to the recommendations of her medical team, who consistently advise her to rest. I believe she is following their instructions, and she will do her best when she is able."

The Associated Press contributed to this report.