Princess Diana once said she thought Prince Charles was joking when he proposed to her

• Princess Diana once told biographer Andrew Morton that she laughed when Charles proposed to her.
• Realizing Charles was "deadly serious," Diana accepted with a simple "OK," she once told Morton.
• Diana also said she and Charles had met just 13 times before he popped the question in 1981.

In secret interviews used for royal biographer Andrew Morton's book "Diana: Her True Story - In Her Own Words," Diana admitted to only ever meeting Charles a total of 13 times before he popped the question in February 1981.

However, some may not be aware of just how taken aback the princess was when Charles proposed.

According to tape transcripts released in the 2017 version of Morton's biography of Diana, the late princess initially thought the whole thing was a joke.

"I went to Windsor and I arrived about 5 o'clock and he sat me down and said: 'I've missed you so much.' But there was never anything tactile about him," Diana was quoted telling Morton in his book. "Anyway, so he said: 'Will you marry me?' and I laughed. I remember thinking: 'This is a joke,' and I said: 'Yeah, OK,' and laughed."

Morton's book, originally published in 1992, says the proposal took place in the nursery at Windsor Castle and that Charles did not get down on one knee.

Speaking on tape to Morton, Diana described the moment the tone of their conversation shifted: "He was deadly serious. He said: 'You do realize that one day you will be Queen.' And a voice said to me inside: 'You won't be Queen but you'll have a tough role.'"

In the tapes, Diana recalled Charles using the now-famous "Whatever in love means" line when the couple were first interviewed by the press about their engagement.

After accepting his proposal with a simple "OK," Diana recalled saying "I love you so much, I love you so much," to which she told Morton Charles replied, "Whatever love means."

The moment recaptured the attention of many after it was re-created on season four of Netflix's "The Crown."

By her own account, documented in Morton's book, Diana had not dated anyone before Charles and had nothing to compare her relationship to at the time.

"I didn't have anything to go by because I had never had a boyfriend. I'd always kept them away, thought they were all trouble - and I couldn't handle it emotionally," she told Morton.

While in the interview tapes Diana described her parents as being "thrilled" at the news, she said her brother, Charles Earl of Spencer, who was 16 at the time, responded to her saying she was engaged by asking: "Who to?"

In the same tapes, Diana would go on to describe her wedding day as "the worst day of her life."

Buckingham Palace did not immediately respond to Insider's request for comment.