Queen Elizabeth II knew son King Charles' marriage to Princess Diana was headed south before they officially split.
In a new interview, Ingrid Seward discussed her book My Mother and I, which details the relationship between the matriarch and her eldest son.
According to the royal expert, Diana — who lost her life in a car crash in 1997 — was the one who used to complain about their marital strife.
"Diana used to go to her private room in between appointments that the queen had, which were every 20 minutes, and burst into tears," Seward shared. "[She would say], ‘Everybody hates me mama, and I hate my husband. He’s a nightmare.’"
"The queen would just stand there [horrified], and Diana would be getting more and more hysterical," Seward said. "[The queen] didn’t know how to handle it, but she thought Charles should know how to handle it. That was a very low point in the relationship."
The author noted that the late monarch — who died in 2022 — "couldn't understand how" Charles, who was a decade older than his ex-wife, "couldn't handle" the situation.
"And [the queen] didn’t understand because she didn’t have the experience to understand something like that. Remember the cloistered world of the royal family, especially in those days," the writer explained. "They didn’t have to ever deal with moral conflict because there was always someone else to do it for them."
"If you didn’t want to talk to someone, the switchboard at Buckingham Palace would just not put them through. So, you never had to take on things. And the queen wasn’t used to doing this," she stated.
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Seward said that from an outsider's perspective, it's easy to understand why the union failed.
"Charles wasn’t a very mature man. She was incredibly immature. They were completely unsuited," the author insisted. "Diana said many, many, many years later to me, ‘If only we’d met at a different time in our lives.’ Diana was 19 when she met Charles, and she was a very naïve 19. She had a fixed idea in her mind of what she thought Charles was, and he wasn’t at all."
The couple married in 1981 and welcomed Prince William and Prince Harry before their divorce was finalized in 1996. Charles also confessed to having an affair with Camilla Parker Bowles, his now-wife.
Fox News Digital spoke with Seward.