Princess Diana wasn't allowed to call Prince Charles by his first name until they were engaged

• Princess Diana was only permitted to address Prince Charles as "sir" until they were engaged.
• The princess wasn't supposed to call him by his first name, writes royal biographer Andrew Morton.
• "In Prince Charles' circle this was considered the norm," Morton added.

It's been 40 years since Prince Charles and Lady Diana Spencer announced their engagement on February 24, 1981.

The couple's engagement interview was watched by royal fans worldwide, but some people may not know how formal the couple had been with one another until that moment.

According to Princess Diana's biography, "Diana: Her True Story," by Andrew Morton, she was required to call Charles "sir" when they were dating and was only allowed to address him by his first name when they became engaged.

"It was not until Lady Diana was formally engaged to His Royal Highness The Prince of Wales that she was given permission to call him 'Charles.' Until then she had demurely addressed him as 'Sir,'" Morton wrote in the book, which was first published in 1992.

While Diana also had an official title - she was formally known as Lady Diana until she got married - Charles used her first name.

"He called her Diana," Morton added. "In Prince Charles' circle this was considered the norm."

Morton went on to explain that Diana's older sister, Lady Sarah, had been just "as formal" during her nine-month-long relationship with the prince.

"It was obviously right to do so because I was never corrected," Lady Sarah told Morton.

Diana met Charles when she was 16 and the prince was 29. However, they didn't start a romantic relationship until three years later. The couple met a total of 13 times before they got married, the princess later said in secretly recorded tapes for her biography.

In the same tapes, Diana famously called her royal wedding the "worst day of my life."

"I don't think I was happy. I never tried to call it off, in the sense of really doing that, but I think [it was] the worst day of my life," she said.

She also described herself as being "a lamb to the slaughter."

Footage from the tapes was published for the first time in the 2017 National Geographic documentary, "Diana: In Her Own Words."