Princess Diana's private letters to the Queen were burned.

According to royal biographer William Shawcross, Princess Margaret burned thousands of letters from the late Princess of Wales addressed to Queen Elizabeth II "because they were so private." The Queen's sister did this shortly after Prince Charles and Princess Diana announced their split.

"Princess Margaret was engaged on one of her periodic 'sortings' of her mother's papers," Shawcross said (as quoted by Express). "On the Princess's orders, large black bags of papers were taken away for destruction rather than for ultimate consignment to the Royal Archives."

"There is no record of just what was thus lost but Princess Margaret later revealed that among the papers she had destroyed were letters from the Princess of Wales to Queen Elizabeth – because they were so private," the biographer added. "No doubt Princess Margaret felt that she was protecting her mother and other members of the family. It was understandable, although regrettable from a historical viewpoint"

Princess Margaret reportedly treated Princess Diana and Sarah Ferguson cruelly after their separation from Prince Charles and Prince Andrew, respectively.

In BBC's documentary "Princess Margaret: The Royal Rebel," Craig Brown revealed how Princess Margaret disapproved Princess Dianas secret interviews about her marriage with Prince Charles.

"She was always very, very cross with people like Diana or Fergie who got into scandals and so she would never have done a secret interview with Panorama or anything like that," Brown said

Princess Margret was furious when she learned about it. So, she wrote a "wounding and excruciating" letter to the People's Princess. Princess Diana confirmed to Andrew Morton the letter in her "Diana: Her True Story - In Her Own Words" biography where she admitted that she received a "furious note of reproof" from Princess Margaret.

On the other hand, the Duchess of York sent Princess Margaret flowers shortly after her split from Prince Andrew to maintain their friendship. However, the Queen's sister wasn't as warm as Ferguson.

"The Duchess of York had sent flowers to Princess Margaret with a little note, no doubt trying to renew their once good friendship and the flowers had been sent straight back with a note from Princess Margaret saying 'how dare you send me flowers, have you ever considered what damage you have done to the Royal Family?'" Wade revealed.

"And I thought that was the worst example of a pot calling a kettle black in history because in the 50s and 60s Princess Margaret was notorious for her behaviour, it was totally out of keeping with her position."

Princess Diana and Prince Charles
Princess Margaret burned Princess Diana's private letters to Queen Elizabeth II for a reason. Pictured: Prince Charles and Princess Diana at Westminster Abbey, London, for a centenary service for the Royal College Of Music on Feb. 28, 1982. Getty Images/Fox Photos