The Queen once referred to Camilla as 'that wicked woman,' according to a royal expert

• Royal expert Katie Nicholl said Queen Elizabeth II once referred to Camilla as "that wicked woman."
• For years, the Queen refused to sanction Camilla and Charles' relationship after their affair.

Seven months before her death, Queen Elizabeth II shocked many when she announced that Camilla, the then-Duchess of Cornwall, would be known as Queen Consort when Charles ascended the throne.

It was a major shift from the monarchy's original plans. In 2005, Buckingham Palace announced that Camilla would become Princess Consort, not queen. But royal experts told Insider the surprising decision was Queen Elizabeth's way of bolstering support for Camilla.

However, Camilla and Elizabeth's relationship wasn't always so strong, according to royal expert Katie Nicholl.

In her book "The New Royals," which was released in October, Nicholl speaks to British historian Robert Lacey about the Queen's feelings toward Camilla in the 1990s.

"In 1998, when Charles was trying to persuade his mother to be more accommodating of Camilla, the Queen described her as 'that wicked woman,'" Lacey said. "Those damning words were reported in distress by Prince Charles himself."

The history of Charles and Camilla's love up to that point was already long, scandalous, and complicated. They had started as young lovers in their 20s in 1970 and were cheating on their respective spouses with each other by the '80s. The royal family was publicly embarrassed in January 1993 when conversations between the future king and Camilla were published by the press — including the infamous moment when Charles said he wanted to be her tampon. A year later, Charles admitted to his adultery during a TV interview.

"If you're looking for one problem or set of problems in the reign of Elizabeth II, they all come down to Charles — and specifically his infatuation and love for one woman, who was not his wife," Lacey told Nicholl.

The ongoing scandals of the '90s — many of which have been depicted in this season of "The Crown" — were a constant source of anguish for the once stable monarchy. Things were so bad that, according to Nicholl, the Queen "would not have Camilla's name uttered in her presence."

But after Charles and Diana officially divorced in 1996, Nicholl said plans were put in place to try and legitimize their relationship in the eyes of the public. Mark Bolland was hired for "Operation Mrs. PB," as it was called, and the public relations expert told Nicholl about his initial strategy.

"At this stage, there was absolutely no talk of marriage or Camilla being queen," he said. "It was more, 'Can we actually just spend some more time together? Can we go to the theater together and not live in fear of terrible headlines?'"

Charles threw Camilla a lavish 50th birthday in July 1997. A month later, Princess Diana died in a tragic car accident. All plans to make Camilla and Charles' relationship more public immediately came to a halt.

The couple continued to keep their relationship a secret until November 1998, when Charles threw a big 50th birthday party and made Camilla his guest of honor, according to Nicholl.

"The Queen stayed away, unwilling to sanction her son's new relationship," Nicholl wrote.

Even without the Queen's approval, the couple forged forward. In January 1999, Charles and Camilla made their first public appearance together during her sister's 50th birthday party at the Ritz Hotel in London. Months later, Charles and Camilla took their children on a joint trip to Greece.

Nicholl said things slowly started to change between Camilla and the Queen. In 2000, Queen Elizabeth attended a 60th birthday party that the couple hosted for King Constantine of Greece. When the Queen invited Camilla to join the royal family for a Golden Jubilee celebration at Buckingham Palace in February 2002, it was seen "as an official end to hostilities," Nicholl wrote.

A year later, the couple moved in together. And in 2005, Charles and Camilla finally got married.

The Queen gave a toast at the couple's reception at Windsor Castle, speaking of the "terrible obstacles" they had overcome.

"I'm very proud and wish them well," she said. "My son is home and dry, with the woman he loves."