Intimate details of Princes William and Harry are revealed in letters penned by their late mother, Diana, Princess of Wales. They form part of a collection of personal Royal artefacts being sold at auction.

In one of the handwritten correspondences – now glimpsed by the public for the first time – Princess Diana described how a young Prince William gave his baby brother "an endless supply of hugs and kisses" while in a later epistle she explained that Prince Harry was "constantly in trouble" at boarding school.

The tender exchanges, written in flowing black ink on Kensington Palace headed paper, were sent by Diana to her personal friend Cyril Dickman in the 1980s and 1990s. They are now to go on sale to the highest bidder for an estimated price of £15,000 ($18,500).

In one, dated 20 September 1984, Diana wrote: "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! 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!"

In another from 17 October 1992 she said: "The boys are well and enjoying boarding school a lot, although Harry is constantly in trouble!"

Dickman has been described by Cheffins, the auctioneer, as "a favourite of every member of the Royal Family". He was a palace steward for more than 50 years. Diana wrote that he had been "so very kind" to 'have sent such a lovely card' in one of the letters."

John Foster of Cheffins auctioneers in Cambridge told the BBC: "One of the nice things about this collection is that it reveals the personal side. It's not just thanking someone for an event, it's things like the handwriting and the squiggly drawings.

"There is also mention of when Diana's father passed away and how sad it was, talking of the children when they were at school, how much trouble they got into, just those personal touches which you never really get to hear about."

princess diana
Princess Diana's letters always attract huge interest at auction RR Auction