Queen Camilla used to be one of Kate Middleton’s “fiercest critics” — and “did object” to her marrying Prince William.

King Charles’ wife believed the now-Princess of Wales, 44, was “too common” to wed William in 2011, according to Christopher Andersen’s “Kate!” biography, out last week.

“She did not think she was up to snuff, as it were,” the new book reads. “She was below the salt. She had no aristocratic blood.”

He claimed Camilla “always saw herself as the mistress of a king, not a queen” and “picked [Princess] Diana to be Charles’ bride” in 1981.

Anderson noted that “the palace didn’t really want” Middleton, whom William met attending the University of St Andrews in Scotland.

“People like Camilla didn’t want her because they felt that she was too common to be the wife of a future king,” the biographer wrote, alleging the 78-year-old “did object” to her daughter-in-law’s “working-class roots.”

Kate’s parents, Carole Middleton and Michael Middleton, are former British Airways employees who founded a successful party supply business.

“[Camilla] was very cognizant of the fact that a future king of England should have, she believed, a marriage to a royal personage, or at least a British aristocrat,” Anderson wrote — not “a descendant of coal miners whose mother had grown up in public housing and once worked as a flight attendant.”

Camilla allegedly believed Carole to be a “gauche opportunist” and “knew a schemer when she saw one.”

Reps for Camilla and Kate did not immediately respond to Page Six’s request for comment.

Kate and William, 43, got engaged in October 2010 and tied the knot in April of the following year.

Ahead of the 2011 nuptials, Camilla and Charles, 77, reportedly “offended” the bride by requesting she change the letter of her first name, Catherine, from a C to a K.

They believed another C royal cypher — a personalized monogram or emblem representing a royal family member — would be “overkill,” Anderson wrote.

When William was left allegedly “fuming” over the “insult,” the subject was dropped.

The author claimed Kate and William’s early days included “a lot of sniping from the sidelines, much of it coming from Camilla’s camp” — but the princess “never put a foot wrong.”

Kate and the Prince of Wales have since welcomed three children — Prince George, 12, Princess Charlotte, 11, and Prince Louis, 8.