Princess Diana's letters
Britain's Princess Diana holds Prince Harry during a morning picture session at Marivent Palace, where the Prince and Princess of Wales are holidaying as guests of King Juan Carlos and Queen Sofia, in Mallorca, Spain August 9, 1988. Reuters/Hugh Peralta

Six letters, handwritten by Princess Diana and addressed to ex-Buckingham Palace steward Cyril Dickman, have been sold at an auction for over £15,000 (AU$25,000).

The notes, which have been described as "private letters between a trusted butler and the royal family" were put up on sale at Cheffins auction house in central London.

One of those candid letters from the princess revealed that Prince Harry was “constantly in trouble at school,” while another described how Prince William flooded Harry with “an endless supply of hugs and kisses” when he was a newborn.

Diana, in her letter written five days after Harry was born in 1984, said it was a “particularly happy time.” “William adores his little brother and spends the entire time swamping Harry with an endless supply of hugs and kisses, hardly letting the parents near!” she detailed.

As stated in a BBC News report, the pile of letters is a part of about 40 lots from the butler’s former estate. The estate sold for around £55,000 (AU$93,000), according to the report. The lot also included Christmas cards, a piece of cake from the Queen Elizabeth II's 1947 wedding, and a note sent from the queen herself.

In one of the letters dated Sep. 20, 1984, the late Princess expressed her gratitude to Mr Dickman for a “lovely card” sent to her on the occasion of her youngest son’s birth.

“The reaction to one tiny person's birth has totally overwhelmed us and I can hardly breathe for the mass of flowers that are arriving here!” Diana wrote in her letter, which sold for around $4,600, with a bidding price of $600-800.

Dickman served the Buckingham Palace for over 50 years and was a much-respected figure by the time he retired in 1992, according to an SBS News report. He has also been credited with helping Diana deal with her father’s death the same year.

Mr Dickman, as described by Cheffins, a Cambridgeshire auction house in the UK, as “a favourite of every member of the Royal Family,” is said that he received akin letters from several other royals too, dating back over 30 years. Cheffins, about the letters, said those were “a unique collection of royal memorabilia.”

Cheffins auctioneers said there had been an international interest. Hundreds of bidders from Australia, Japan and the US have been reported as trying to buy the items.

A Sky News report stated auctioneer Luke MacDonald as saying, “They give a more personal insight into the lives of the royals (that) perhaps we can relate to far more than the more formal times we see them in the press and on television.”

“Her Royal Highness Princess Diana was adored by so many people,” MacDonald added. “I think people feel that to actually have a letter, handwritten by her - it's something very special.”