Violet Collison, who the Princess of Wales affectionately called Collie, was head housekeeper to her parents at Park House on the Sandringham Estate in Norfolk, where Diana spent her childhood years.

Most of the letters from Diana are thank-you notes for birthday and Christmas presents given to the princess and her children, William and Harry.

The cards, which often include a line about Diana's life at the time, are expected to sell for thousands of pounds.

In one letter, written from Kensington Palace on 25 September, 1984, Diana thanked Collie for a gift to Harry.

She said: "William adores his little brother and spends the entire time pouring an endless supply of hugs and kisses over Harry."

A double-sided letter, written on Buckingham Palace notepaper, was sent three weeks before the royal wedding.

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In the note, the princess wrote: "Everyone frantically busy here doing last minute decorations… the bride-to-be has remained quite calm!"

Collie followed Diana's mother Frances Shand Kydd to London in 1967 when her marriage ended. The housekeeper worked for Frances and her new husband until her retirement in 1973.

Collie died in 2013 at the age of 89.

The items are being auctioned by Sworders in Stansted Mountfitchet in Essex on 30 July.